RELATED: Obama, celebs support Kimmel after son's health scare
During an emotional and heartfelt monologue, Kimmel explained, "They found that Billy was born with a heart disease."
The late night talk show host took last week off, after telling viewers about his son's open-heart surgery. The family says Billy is doing great.
On Monday, the "Jimmy Kimmel Live!" team tweeted a sweet photo of Kimmel laughing during rehearsal at the desk, surrounded by mariachi musicians. They wrote, "He doesn't like when we make a fuss, so we did anyway..."
One of Kimmel's guests for Monday night is Louisiana Senator Bill Cassidy.
The republican said a health care bill should pass the "Kimmel test" and cover pre-existing conditions.
Watch "Jimmy Kimmel Live!" weeknights at 11:35 p.m. after Eyewitness News News at 11 p.m.