Jimmy Fallon handily took the gold medal for non-Olympic television programming Monday when he made his debut appearance as host of The Tonight Show succeeding Jay Leno.
Twenty per cent of households in 56 local markets watching television at midnight tuned in to Fallon, according to preliminary Nielsen ratings data shared by NBC on Tuesday.
Complete national viewership figures are still pending, but entertainment trade journal Variety said the initial data suggests as many as 10 million Americans stayed up to watch the Fallon premiere.
That doesn't take into account those who went onto the internet on Tuesday to catch segments posted on YouTube or on the Hulu video streaming website co-owned by NBC.
NBC said the show - which started a half-hour later than usual due to Winter Olympics coverage - was "easily the number-one non-Olympic telecast" on Monday, as well as the evening's dominant late-night program.
Trailing well behind were CBS's The Late Show with David Letterman and ABC's Jimmy Kimmel Live each with five per cent of households tuned in.
Leno, 63, signed off from The Tonight Show after 22 years on February 6 as NBC relocated the show from Los Angeles to New York and put Fallon, 39, in the host's chair in a bid to attract a younger, hipper demographic.
"I really don't know how I got here," said Fallon to a cheering studio audience after passing through a blue curtain to take the reins of one of the greatest franchises in American television.
"I just want to do the best I can, and take care of the show for a while," he added.
"If you guys let me stick around long enough, maybe I'll get the hang of it."
His guests on Monday night's show included rapper/actor Will Smith and Irish rock band U2, performing atop Rockefeller Center, plus a parade of celebrities including Lady Gaga, Lindsay Lohan, Mike Tyson and Robert De Niro who paid back mock $US100 bets that he'd never host the show.
