Jay Leno performs in what is being billed as Jay's Comedy Stimulus Plan at the Palace in the Detroit suburb of Auburn Hills, Mich., on April 7.Jay Leno performs in what is being billed as Jay's Comedy Stimulus Plan at the Palace in the Detroit suburb of Auburn Hills, Mich., on April 7. (Carlos Osorio/Associated Press)

Jay Leno, who's soon to leave the Tonight Show, became ill Thursday, and the taping of his show was cancelled.

His spokesman, Dick Guttman in Burbank, Calif., said Leno left his office at NBC's studios about midday.

He checked himself into nearby Providence Saint Joseph Medical Center for observation. Guttman characterized Leno's ailment, which he couldn't identify, as "mild."

Scheduled guests on Thursday's show were Ryan Reynolds, Jules Sylvester and the swing band Big Bad Voodoo Daddy. Instead, NBC planned to air a rerun.

It was an unusual lapse for the famously intrepid performer, who routinely fills off-days from his TV show with live appearances on the comedy circuit.

Even feeling sick, he made phone calls and wrote jokes on Thursday, Guttman said.

Leno, who turns 59 on Tuesday, will leave the Tonight Show on May 29 after 17 years. But he will continue on NBC, with a Monday-through-Friday program at 10 p.m., starting in the fall.

The top-rated late-night host's move to prime time created a stir in the industry, taking the time slot usually reserved by broadcast networks for dramas such as ER.

And Leno has continued to make news, scoring a coup by booking U.S. President Barack Obama as a guest and performing free comedy concerts in the recession-ravaged Detroit area.