One week after Peter Cullen’s death, new information has revealed what caused the legendary voice actor’s passing.

Cullen, whose unmistakable voice turned Optimus Prime into one of the most recognizable characters in popular culture, died at his Los Angeles home on Aug. 26, 2026. He was 85.

At the time his death was announced, his family said he had passed away peacefully while surrounded by loved ones, but no detailed medical explanation was immediately made public.

That changed on Sept. 2.

According to Cullen’s death certificate, he died from cardiac arrest. The document identifies respiratory arrest as the underlying cause and lists several other significant medical conditions that contributed to his death, including cerebellar metastasis, hypoxia, chronic obstructive pulmonary disease, hypertension and hyperlipidemia.

The new details provide a medical answer to a question fans had been asking since the beloved actor’s death was announced.

But for many of those fans, Cullen’s legacy remains tied to something much larger than the circumstances of his final days.

For more than four decades, he was the voice that made Optimus Prime sound not merely powerful, but trustworthy.

His Family Said He Died Peacefully

Cullen’s family described his final moments in deeply personal terms.

They said he passed away peacefully, surrounded by his family and remembered by the many friends and fans whose lives he had touched.

The family also made an unusual request about how they hoped people would honor him.

Rather than focusing only on his television and movie work, they encouraged fans to serve their own communities and to lead others with compassion, integrity and loyalty.

Their message ended with words that have become closely connected to Cullen and the character who defined his career:

“Be strong enough to be gentle.”

It was a fitting farewell for the man whose performance as Optimus Prime helped turn a transforming robot into a symbol of leadership for generations of viewers.

Optimus Prime Changed Peter Cullen’s Life

Cullen began voicing Optimus Prime in 1984 when The Transformers animated series introduced the heroic Autobot leader to television audiences.

The role could easily have been played as a conventional action character.

Optimus was enormous.

He was powerful.

He commanded warriors and fought a seemingly endless battle against the Decepticons.

But Cullen gave the character something unexpected.

Calm.

Warmth.

Authority without arrogance.

Optimus could sound intimidating when necessary, but the performance was rarely built around aggression alone. Cullen’s deep voice made the character sound like someone others would willingly follow.

That interpretation became so closely associated with Optimus Prime that it survived dramatic changes in the franchise itself.

The original cartoon ended decades ago.

The toys changed.

Animation styles changed.

New generations of children discovered Transformers.

Eventually, the franchise became a massive live-action movie property.

Cullen remained.

He Returned for the Live-Action Movies

More than two decades after first voicing Optimus, Cullen returned to the role for Michael Bay’s 2007 live-action Transformers film.

For longtime fans, hearing that voice again helped connect the new blockbuster version of the franchise to the animated series they had grown up watching.

Cullen subsequently continued voicing Optimus across films, animation and video games.

His final movie appearance as the character came in 2023’s Transformers: Rise of the Beasts.

By then, the relationship between Cullen and Optimus had become extraordinarily rare in entertainment.

Actors are routinely replaced when franchises reboot.

Characters are redesigned.

Studios search for new interpretations.

Yet Cullen’s voice became so fundamental to Optimus that audiences continued wanting him back decades after his first recording session.

For many fans, Optimus Prime simply sounded like Peter Cullen.

Optimus Wasn’t His Only Famous Voice

Although Optimus became Cullen’s defining role, his voice had been part of another beloved fictional universe for decades.

Beginning in 1988, Cullen voiced Eeyore in The New Adventures of Winnie the Pooh.

It was a remarkably different performance.

Optimus Prime was a commanding leader capable of delivering heroic speeches before battle.

Eeyore was quiet, gloomy and perpetually resigned to the small disappointments of life in the Hundred Acre Wood.

Cullen continued portraying Eeyore in several projects, including The Book of Pooh, Piglet’s Big Movie and The Tigger Movie.

The contrast between the characters demonstrated the range behind a voice many people primarily associated with giant robots.

One role could inspire audiences with courage.

The other could make them laugh with a tired sigh.

Cullen made both instantly recognizable.

He Voiced More Than One Transformer

Even within the original Transformers cartoon, Cullen’s work extended well beyond Optimus Prime.

He also provided voices for characters including Ironhide, Streetwise, Wingspan, Slugslinger and Nightstick.

Voice actors working in animation frequently portray several characters within the same production, sometimes requiring them to create completely different personalities during the same recording sessions.

But none of Cullen’s other Transformers roles ultimately eclipsed Optimus.

That character became his signature.

For children watching the series in the 1980s, his voice became inseparable from the idea of a leader who could be powerful without being cruel.

Decades later, many of those children became adults who introduced the franchise to their own kids.

Cullen’s voice followed them across generations.

The Medical Details Answer One Question

The release of Cullen’s death certificate now provides clarity about his passing.

Cardiac arrest was listed as the immediate cause of death, with respiratory arrest identified as the underlying cause.

The document also recorded several contributing medical conditions.

Those details explain more than fans knew when Cullen’s death was first announced.

But they do not change the central fact his family emphasized from the beginning: Cullen died peacefully at home in Los Angeles with loved ones around him.

He is survived by four children.

For a performer whose career was largely built behind a microphone rather than in front of a camera, the scale of the public reaction to his death demonstrated just how deeply a voice can become part of people’s memories.

“Be Strong Enough to Be Gentle”

There may be no more appropriate phrase with which to remember Peter Cullen.

Optimus Prime was physically powerful enough to defeat almost anything placed in front of him.

What made Cullen’s version special was that power never seemed to be the character’s most important quality.

His Optimus sounded patient.

Protective.

Measured.

Even when the character was preparing for battle, Cullen gave audiences the sense that violence was a responsibility rather than something Optimus enjoyed.

That interpretation helped turn a character originally created to accompany a toy franchise into something far more enduring.

Children heard a robot.

Adults looking back often remembered a model of leadership.

Cullen gave Optimus both.

Now, seven days after his death at 85, the medical circumstances surrounding his passing have become clearer.

But the words his family chose for his farewell may ultimately say more about his legacy than anything written on a death certificate.

Serve others.

Lead with compassion, integrity and loyalty.

And always remember to be “strong enough to be gentle.”

For millions who grew up hearing Peter Cullen speak through Optimus Prime, it sounds like one final message from a voice they will never forget.