In the years 1980-1989, 78 patients with an acute anterior cruciate ligament (ACL) rupture sustained during downhill or cross-country skiing were treated at the University Hospital of Tampere, Finland. In every case, the ACL rupture was verified at arthroscopy or open surgery. The injury mechanism could be clarified for 51 patients using a collection of pictures of the most typical injury mechanisms in skiing. Thirty-nine of them (76%) were women and 12 men (24%). In 32 cases (63%) the injury occurred during downhill skiing and in 19 cases (37%) during cross-country skiing. In 24 cases (47%) the injury mechanism was valgus-external rotation, in 21 cases (41%) flexion-internal rotation, in two cases hyperextension-internal rotation, while in four cases the exact mechanism remained unclear. The great majority of the patients with an injury mechanism of flexion-internal rotation were women (90%), and they were significantly older than the patients with an injury mechanism of valgus-external rotation (mean ages 44 and 34 years, respectively; P < 0.05). According to the patients' subjective evaluation, the main reasons for the injury were poor ski area conditions (such as slippery slopes and trails) and deficient equipment, especially poorly functioning bindings. Many of them had had little skiing experience before the accident.