Robert Luketic's real-life drama 21 has gone straight in at the US box office top spot.

The story of six MIT students who won millions at Vegas casinos knocks the Dr Seuss adaptation Horton Hears A Who! into second spot.

Other new entries come from Leslie Nielson's spoof comedy Superhero Movie at third and Ryan Phillippe's drama Stop-Loss, which lands at number eight.

The top ten in full:

1. (-) 21 - $23,700,000
2. (1) Horton Hears a Who! - $17,425,000
3. (-) Superhero Movie - $9,510,000
4. (2) Tyler Perry's Meet the Browns - $7,760,000
5. (4) Drillbit Taylor - $5,800,000
6. (3) Shutter - $5,325,000
7. (5) 10,000 B.C. - $4,880,000
8. (-) Stop-Loss - $4,525,000
9. (7) College Road Trip - $3,505,000
10. (8) The Bank Job -$2,800,000