The purpose of this study was to examine gender differences in the incidence of anterior cruciate ligament (ACL) injuries in a population of high-level team handball players. We also wanted to examine injury mechanisms and possible risk factors for ACL injuries, including menstrual status. The study was done prospectively during the 1993-94, 1994-95, and 1995-96 seasons. We found 28 ACL injuries, 23 among women (incidence:​ 0.31±0.06 injuries per 1000 player hours) and 5 among men (0.06±0.03 inj./1000 h;​ P<0.001 vs women;​ risk ratio:​ 5.0). Of the 28 injuries, 24 occurred during competition (0.91±0.19 inj./1000 h;​ women:​ 1.60±0.35 inj./1000 h;​ men:​ 0.23±0.13 inj./1000 h;​ P<0.001 vs. women;​ risk ratio:​ 7.0) and 4 during training (0.03±0.02 inj./1000 h;​ P<0.001 vs. competition;​ risk ratio:​ 29.9). Nearly all the injuries (n=25) occurred in non-contact situations when the players performed high-speed plant-and-cut movements which they were well accustomed to. A reliable menstrual history could be obtained in 17 of the 23 cases among females. Five of the injuries occurred in the menstrual phase, 2 in the follicular phase, 1 in the early luteal phase and 9 in the late luteal phase (chi-square_{3 d.f.}=13.2;​ P<0.01). The results suggest that there may be an increased risk of ACL injury during the week prior to or after the start of the menstrual period.