LOS ANGELES — Movie-goers laid their money down on "21," a gambling romp that was the weekend's box-office high-roller with a $23.7 million debut, according to studio estimates Sunday.

Starring Kevin Spacey, Kate Bosworth and Jim Sturgess as math geniuses who make a killing at Vegas' blackjack tables, Sony's "21" bumped off "Dr. Seuss' Horton Hears a Who!", which had been No. 1 the previous two weekends.

"Horton Hears a Who," distributed by 20th Century Fox, slipped to second-place with $17.4 million, raising its total to $117.3 million. It was the first movie this year to pass the $100 million mark.

Despite solid holdover crowds for "Horton," overall business continued to dip. The top-12 movies took in $90 million, down 23 percent from the same weekend last year, when "Blades of Glory" was No. 1 with $33 million.

Hollywood started 2008 with a strong uptick in January, but revenues have trailed off steadily since. Movie admissions, which had been up as much as 10 percent in early February, now are 2.6 percent behind 2007's, according to box-office tracker Media By Numbers.

By this time last year, Hollywood already had churned out a blockbuster with "300," which eventually topped $200 million, and other hits that included "Wild Hogs" and "Meet the Robinsons."

"Last year was very, very strong at this point. It's made comparisons very tough," said Paul Dergarabedian, president of Media By Numbers. "We're not that far into the year, so every down weekend has a huge impact on the bottom line."

This weekend's other new movies had so-so openings. Debuting at No. 3 with $9.5 million was the MGM-Weinstein Co. release "Superhero Movie," a spoof of one of Hollywood's favorite action genres.

Paramount's "Stop-Loss," starring Ryan Phillippe as an Iraq War veteran sent back against his wishes for another tour of duty, opened at No. 8 with $4.5 million.

Former "Friends" co-star David Schwimmer's directing debut, the comedy "Run, Fat Boy, Run," was a dud, taking in $2.4 million to finish out of the top 10. Released by Picturehouse, the movie stars Simon Pegg as an out-of-shape slacker running a marathon to impress his former fiance (Thandie Newton).

The audience for "21" was split almost evenly between men and women and those older and younger than 25, according to Sony.

"One of the coolest things is it really did play to everyone. It got male, female, young, old," said Rory Bruer, Sony's head of distribution.

Estimated ticket sales for Friday through Sunday at U.S. and Canadian theaters, according to Media By Numbers LLC. Final figures will be released Monday.

1. "21," $23.7 million.

2. "Dr. Seuss' Horton Hears a Who!", $17.4 million.

3. "Superhero Movie," $9.5 million.

4. "Tyler Perry's Meet the Browns," $7.8 million.

5. "Drillbit Taylor," $5.8 million.

6. "Shutter," $5.3 million.

7. "10,000 B.C.", $4.9 million.

8. "Stop-Loss," $4.5 million.

9. "College Road Trip," $3.5 million.

10. "The Bank Job," $2.8 million.