Leno and Letterman Go Head to Head

After a smashing return, Jay Leno appears headed to a battle with David Letterman for late-night television supremacy.

The Nielsen Co. says Leno averaged 5.6 million viewers last week, his first back at NBC's "The Tonight Show" following the ill-fated prime-time experiment. Letterman had 3.7 million at CBS. Leno took advantage of curiosity over his return and a strong lineup of guests, including Olympic stars and Sarah Palin. It was a half-million more than he averaged his last season at "Tonight."

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It's been much closer the second week. Through four nights, Leno is averaging 4.3 million viewers and Letterman has 4 million.

Considering Leno usually beat Letterman by a million and a half during their last season of competition, that's a sign of erosion for NBC's late-night franchise. Letterman consistently beat Conan O'Brien after O'Brien took over last year.