• Los Angeles Times, Thursday, June 11, 2009 9:30 AM

After a strong start in the ratings, "The Tonight Show With Conan O'Brien" is already slipping behind "Late Show With David Letterman," the program that Jay Leno defeated handily for years.

On Tuesday night, Letterman edged past O'Brien in the overnight household ratings for the first time since O'Brien took over his show, per Nielsen. More troubling for NBC, the "Tonight" numbers since late last week have been well below Leno's overnight average during the first three months of this year. Letterman's numbers, meanwhile, are landing within his usual range.

Those figures don't tell the whole story, though. While Leno's audience skewed old for a late-night program, NBC points out that O'Brien is strongest among young adults. It's likely that, even with his declines, O'Brien will still emerge stronger among young people than Leno was. But the real battle is just beginning. Late-night rating trends tend to shift slowly, as viewers develop loyalties for a host over months or even years. When Letterman took over his CBS show in 1993, he initially racked up big victories over Leno, but within a few years emerged as the perpetual runner-up.

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