Menstrual function was assessed comparatively in different categories of 155 Nigerian athletes, aged 13-19 years, and 135 nonathletes, aged 12-18 years, who answered questionnaires and were interviewed. Menstruation was more regular and normal in the nonathletes (44%) than the athletes (21%). Although both groups generally had similar patterns of menstrual dysfunction, secondary amenorrhea or oligomenorrhea (O/A) was more prevalent in the athletes (25%) than the nonathletes (10%). Among the athletes, only the distance runners were significantly lighter and leaner than nonathletes and athletes of other sports (P less than 0.05). Regardless of sports activity, oligomenorrheic subjects had substantially lower body weight relative to stature and were leaner than subjects of the other menstrual categories. In the oligomenorrheic group, a greater proportion of the athletes (59%) and nonathletes (57%) had lean body mass values below the 41st percentile. The premenarche-trained athletes (48%) reported a higher incidence of menstrual cycle irregularities than the athletes who began training after the menarche. Results support the role of body composition-related factors in the etiology of exercise-associated O/A.