What happens in Vegas stays in Vegas -- even at the box office.

Sony Pictures' blackjack thriller "21" topped the weekend charts with an estimated $15.1 million in ticket sales, as the George Clooney-Renee Zellweger football comedy "Leatherheads" from Universal Pictures opened to a soft $13.5 million, apparently edging out 20th Century Fox and Walden Media's "Nim's Island" for No. 2.

Nikki Rocco, Universal's president of domestic distribution, said she was "disappointed with the result" for "Leatherheads," a $58-million production.

Mixed reviews hurt the adult-skewing, PG-13-rated film, she said, as did Saturday's Final Four telecasts in the college basketball championship tournament. The picture got only a slight bump in business on its second day.

Three-fourths of the audience was older than 30, and 54% was female, the studio said. Patrons surveyed by CinemaScore rated the movie a lukewarm C-plus.

"Leatherheads" opened toward the low end of industry expectations, as overall business was down from the same period in 2007 for the third straight weekend, according to data tracker Media by Numbers. Year-to-date box-office revenue is down 2%, while attendance is off 5%.

The relatively modest 37% second-weekend drop for "21," which stars Jim Sturgess, Kate Bosworth, Laurence Fishburne and Kevin Spacey, indicates good word-of-mouth for the film despite its mixed reviews from critics.

The PG-13 thriller, produced for $35 million, has grossed $46.5 million and appears headed for more than $70 million in U.S. ticket sales, said Rory Bruer, Sony's president of domestic distribution.

The PG-rated "Nim's Island," which could end up in the weekend's No. 2 slot when final figures are tallied today, met industry expectations with its estimated haul of $13.3 million. Starring Jodie Foster, Abigail Breslin and Gerard Butler, the family adventure was produced for about $37 million.

The audience, estimated at 60% female and 51% over 25, was well balanced, said Chris Aronson, Fox's senior vice president of distribution. "The fact that we caught a lot of dads and sons is fantastic," he said.

Fox's other family film, "Horton Hears a Who!," added $9.1 million to rank No. 4, bringing its total to $131.1 million. The G-rated Dr. Seuss adaptation is this year's only movie to top $100 million -- a major reason the business has slumped. Last year by this time "300," "Ghost Rider" and "Wild Hogs" had reached that milestone.

The weekend's other wide opener, the R-rated horror film "The Ruins," met expectations with a $7.8-million opening, ranking No. 5. DreamWorks Pictures and Spyglass Entertainment partnered on the $25-million production, released by DreamWorks/Paramount.

Three major releases will compete this weekend: Sony's horror film "Prom Night"; Fox Searchlight's crime drama "Street Kings"; and Miramax's comedy "Smart People."

josh.friedman@latimes.com