Jimmy Kimmel breaks down on TV as he shares his newborn's 'terrifying' health battle.

In a rare change of pace for comedian Jimmy Kimmel, the 49-year-old comedian and late night talk show host opened his latest show with a heartbreaking story.

During the opening monologue of his show Jimmy Kimmel Live!, he revealed that he and his wife, Molly, had welcomed a baby boy on Friday, April 21.

They named him William ‘Billy’ John Kimmel, with Kimmel telling the crowd it was “an easy delivery, six pushes and he was out”.

"He appeared to be a to be a normal, healthy baby until about three hours after he was born," he said.

A "very attentive nurse" soon noticed that Billy had a murmur in his heart.

"It's quite common in newborn babies, but she also noticed he was a bit purple, which is not common," he said.

"My wife and I assumed it was nothing, our daughter had a heart murmur too, and we didn't notice he wasn't the colour he was supposed to be.

Billy and Kimmel were ushered into another room - leaving his wife alone in her room to recover - while the nurse called for help. When doctors began to fill the room, Kimmel said he realised things were getting "serious".

Little Billy wasn't getting enough oxygen in his blood. An X-ray showed his lungs were clear, which meant there was a problem with his tiny heart.

A tearful Kimmel explained to the audience that the discovery was "terrifying".

"My wife is back in the recovery room, she has no idea what's going on and I'm standing in the middle of a lot of worried-looking people...who are trying to figure out what the problem is."

Doctors soon confirmed the worst: Billy was born with a heart disease, meaning his pulmonary valve was completely blocked, and he also had a hole in the wall between the left and right sides of his heart.

Just three days after he was born, Billy underwent open heart surgery to fix one of the two defects in his heart.

"The operation was a success…it was the longest three hours of my life," Kimmel said.

Six days later, the family was able to bring him home.

"He's eating, he's sleeping, he peed on his mother today...he's doing all the things that he's supposed to do."

Showing off a photo of his little boy, Kimmel joked, "poor kid...not only did he get a bad heart, he got my face".

He took the opportunity to thank the team of doctors, his colleagues, his family and yes, his famous friends.

"Even that son of a bitch Matt Damon sent flowers," Kimmel joked.

"I want to thank my wife Molly...for being so strong and level-headed and loving during the worst nightmare a new mother could experience.

"I couldn't ask for a better partner. I’m so happy we had this baby together, I’m definitely getting a vasectomy after this."

His heartfelt story came with a powerful message about America's healthcare system, and Donald Trump's attempts to cut funding for the National Institute of health.

"We were brought up to believe we live in the greatest country in the world, but until a few years ago, millions of us had no access to health insurance at all," Kimmel.

"If your baby is going to die, and it doesn't have to, it shouldn't matter how much money you make. That's something that...we all agree on.

"No parent should ever have to decide if they can afford to save their child's life. It shouldn't happen. Not here."