The most commonly tested hypothetical cause of athletic amenorrhea has been low body fat. Test results have conflicted because of mixed groups of athletes and methodologic problems. In this study, we measured body fat only in distance runners (greater than 53 km X wk-1) of the same somatotype who clearly had regular menses or secondary amenorrhea; this permitted more valid group comparison of body fat using hydrostatic weighing. The regularly menstruating group (N = 7) had 12 periods X yr-1 at intervals of 26.5 +/- 1.0 (SE) days with a duration of 4.1 +/- 0.4 days. In the athletic amenorrhea group (N = 7), menstrual periods had been absent for 1 to 10 yr (average = 3.9 +/- 1.3 yr); they were gynecologically evaluated to restrict the group to those with athletic amenorrhea. The groups were similar in a number of categories: weight, height, age, menarcheal age, weekly training mileage, days/week training, years of training, and maximum oxygen uptake. Percent body fat for the two groups was the same: 17.7 +/- 2.1% for the amenorrheic athletes and 17.4 +/- 1.2% for the regularly menstruating athletes (P = 0.91). These data do not support the idea that low body fat per se causes athletic amenorrhea.