If it wasn't for the presence of Regis Philbin, you might not realize that Friday's show marks a milestone for David Letterman.

It will be 20 years to the day since Letterman made his late-night debut, as host of Late Night on NBC. He moved to CBS in 1993.

Letterman, who rarely gives interviews, will probably mention it only briefly Friday. There will be no prime-time anniversary special.

Yet he can't hide Philbin who has become something of a milestone man on Letterman's Late Show. When Letterman announced he had to undergo heart surgery two years ago, it was on the air to Philbin, and he was also there for Letterman's first day back following his recovery.

Philbin was also the first person Letterman traded jokes with during his first show after the Sept. 11 terrorist attacks. It will be Philbin's 61st appearance on Late Show, second only to Tony Randall's 70.

The anniversary is "an incredibly important achievement, especially in this day and age," said Rob Burnett, one of the show's three executive producers. "When you think of the people who have stayed on television and remained a force on television, you can count them on one hand."

The heart surgery and Letterman's much-praised response to the terrorist attacks last fall gave him more attention after a few years where he was somewhat taken for granted.

"He did a good job after Sept. 11," said Marc Berman, a television analyst for Media Week Online. "He's not the warmest guy in the world, but he certainly showed a different side."

Although he's won four straight Emmy Awards, Letterman is still second banana to NBC's Jay Leno in the late-night ratings. Leno averages 6 million viewers this season on the Tonight show, down from 6.3 million a year ago, according to Nielsen Media Research.

Letterman's viewership of 4.36 million this year has changed little from last year's 4.33 million, but he's up 12 percent among younger viewers, age 18 to 49.

"With him and Jay, one show always seems to have the momentum and one doesn't," Berman said, "and Dave has it."

While it's a punishing schedule Friday's show will mark Letterman's 3,558th broadcast Letterman is grateful that he has the platform, Burnett said.

His contract with CBS expires this summer, and a spokesman said Letterman is in discussion with the network on another deal.

Burnett said he's had a few discussions with Letterman about how long the 54-year-old host wants to keep doing his job, but he really has little more insight than the show's viewers.

"I think Dave is in the rare position of someone who can decide for himself how long his career will go," Burnett said. "That's very unusual in television and sports. It's a decision only he can make."