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 Grand Master.
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 Viktor Korchnoi, and in 1971 lost the semi-final 6 - 0 to Bobby Fischer, who went on to win the title.
In 1988 he lost a game to Deep Thought in the Software Toolworks Championship, becoming the highest FIDE ranked player (at 2560) and the first Grand Master 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. His Elo rating in the October 2003 FIDE list was 2470.
Larsen was 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 Aaron Nimzowitsch's) honour.