Sony's card-counting drama 21 came up trumps at the North American box office this weekend, opening at the top of the domestic chart with an estimated three-day gross of $23.7m.

Last week's leader Horton Hears A Who! held up well in its third weekend, but three other new releases had disappointing debuts, with Dimension/MGM's Superhero Movie and Picturehouse's Run, Fat Boy, Run performing well below expectations.

Opening in 2,648 theatres (for a per-theatre average of $8,950), Columbia Pictures release 21 drew a wide demographic and easily out-performed the weekend's three other PG-13 debutants.

Telling the fact-based story of a group of college students who beat the house in Las Vegas, the film, which stars British up-and-comer Jim Sturgess, Kate Bosworth and Kevin Spacey and is directed by Robert Luketic, got mostly negative reviews. But Columbia won audiences over with a flashy and extensive marketing campaign.

Coming in second, Fox's Horton Hears A Who! was down only 29% in its third weekend, grossing an estimated $17.4m from 3,826 theatres (for a $4,554 per-theatre average). With a total gross so far of $117.3m, the computer animated family comedy became the first release of 2008 to cross the century mark.

Superhero Movie, the genre spoof presented by the Weinstein Co's Dimension label and distributed domestically by MGM, opened in third place, but its estimated gross of $9.5m from 2,960 theatres (average - $3,212) was considerably smaller than expected. Recent spoofs Meet The Spartans and Epic Movie both opened with more than $18m domestically and the Weinstein Co's Scary Movie 4 debuted with $40.2m in 2006.

Produced by Scary Movie series director David Zucker, directed by Scary Movie 4 co-writer Craig Mazin and featuring Drake Bell and Leslie Nielsen, Superhero elicited few laughs from critics and may now lead to a reassessment of the genre spoof genre.

Paramount's Stop-Loss, an R-rated Iraq war-themed drama with Ryan Phillippe starring for director Kimberly Peirce, was admired by critics and opened, in eighth place, with $4.5m from 1,291 screens (average - $3,505).

A marketing campaign that emphasized an attractive young cast rather than a dark story was not enough to stop the film becoming the latest Iraq-related box office casualty.

Run, Fat Boy, Run, Picturehouse's PG-13 comedy starring Simon Pegg and directed, as his feature debut, by David Schwimmer, was an even bigger disappointment, failing to make the top ten with its estimated $2.4m from 1,133 theatres (average - $2,109).

Critics were divided on the film, which was a hit last autumn in the UK, and the take was well down on the $5.8m domestic opening from only 825 screens achieved last year by British star Pegg's Hot Fuzz.

Last week's new wide releases had steep declines in their second weekends. Tyler Perry's Meet The Browns, from Lionsgate, was off 61% to $7.8m, for a total to date of $32.8m. Paramount's Drillbit Taylor was down 44% to $5.8m, for a total of $20.6m. And Fox's Shutter slid 49% to $5.3m, for a $19.1m total.

Holding better were Buena Vista's College Road Trip, which was down only 25% in its fourth weekend, and Lionsgate's The Bank Job, off just 33% in its fourth weekend.

Next weekend's wide releases are: Universal's American football comedy Leatherheads, with George Clooney directing and starring opposite Renee Zellweger; Fox's family adventure Nim's Island, with Jodie Foster, Abigail Breslin and Gerard Butler; and Paramount/Dreamworks' horror thriller The Ruins.

Estimated Top 10 North America March 28-30, 2008
Film (Dist)/Int'l dist/Est wkd gross/Est total to date

1 (-) 21 (Sony) SPRI $23.7m -
2 (1) Dr Seuss' Horton Hears a Who! (Fox) Fox Int'l $17.4m $117.3m
3 (-) Superhero Movie (MGM) TWC $9.5m -
4 (2) Tyler Perry's Meet The Browns (Lionsgate) n/a $7.8m $32.8m
5 (4) Drillbit Taylor (Paramount) PPI $5.8m $20.6m
6 (3) Shutter (Fox) Fox Int'l $5.3m $19.1m
7 (5) 10,000 BC (Warner Bros) WBPI $4.9m $84.9m
8 (-) Stop-Loss (Paramount) PPI $4.5m -
9 (7) College Road Trip (Buena Vista) WDSMPI $3.5m $38.4m
10 (8) The Bank Job (Lionsgate) Arclight Films $2.8m $24.1m