Larsen is considered to be the strongest chess player ever born in Denmark. He has won the Interzonal tournament on 3 occasions (1964 at Amsterdam, 1967 at Sousse in Tunisia, and 1976 at Biel in Switzerland). He became an International Master at the age of 19 in 1954 and two years later gained the rank of International Grandmaster.
In the 1965 Candidates matches he lost in the semi-final to Mikhail Tal, a former world champion. In 1968 he lost the semi-final to Boris Spassky, who went on to win the title, and in 1971 lost the semi-final 0-6 to Bobby Fischer, who also went on to win the title.
In 1988 he lost a game to Deep Thought in the Software Toolworks Championship, becoming the first Grandmaster and the player with the highest ELO rating (by then 2560) to be defeated by a computer in tournament play.
Larsen has continued to play occasionally in tournaments to the present day. In 1999 he finished 7th of 10 in the Danish Championship, and he was 4th in the 2002 Najdorf Memorial knock-out in Buenos Aires. By July 2004 his ELO rating in the FIDE list was 2461.
Larsen was an imaginative player, more willing to try unorthodox ideas than most other top players; and is noted for occasionally employing unusual openings. He is one of the very few modern grandmasters to have employed Bird's Opening (1. f4) with any regularity, and the opening move 1. b3 is sometimes called the Nimzo-Larsen Attack in his (and Aron Nimzowitsch's) honour.
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. d4 b5 7. Bb3 d5 8. dxe5 Be6 9. c3 Bc5 10. Nbd2 O-O 11. Bc2 Bf5 12. Nb3 Bg4 13. Nxc5 Nxc5 14. Re1 Re8 15. Be3 Ne6 16. Qd3 g6 17. Bh6 Ne7 18. Nd4 Bf5 19. Nxf5 Nxf5 20. Bd2 Qh4 21. Qf1 Nc5 22. g3 Qc4 23. Qg2 Nd3 24. Bxd3 Qxd3 25. Bg5 c6 26. g4 Ng7 27. Re3 Qd2 28. b3 b4 29. Qh3 bxc3 30. Qh6 Ne6 0-1