Bone mass and metabolic features were studied in 17 women distance runners. Eleven of the women had secondary amenorrhea for 1 to 7 years. Six women have maintained regular menses since menarche. Both groups were matched for aerobic capacity, body fat, exercise intensity, and age of menarche. Mineral density of lumbar spine in the amenorrheic runners was lower than that in the cyclic women and age-matched controls, but higher than that in runners with secondary amenorrhea who are less physically active. Mineral density of the radius was normal in both groups. Running-related fractures were more frequent in amenorrheic women. Metabolic assessment showed no differences between groups, except that serum triiodothyronine was lower in the amenorrheic group, perhaps reflecting low calorie intake. Intense exercise may reduce the impact of amenorrhea on bone mass; however, amenorrheic runners remain at high risk for exercise-related fractures.