PURPOSE: To examine the effect of oral contraceptives (OC) on body weight, fat mass, percent body fat, and lean mass in young female distance runners. METHODS: The study population consisted of 150 female competitive distance runners aged 18-26 yr who had participated in a 2-yr randomized trial of the effect of the OC Lo/Ovral (30 microg of ethinyl estradiol and 0.3 mg of norgestrel) on bone health. Weight and body composition were measured approximately yearly by balance beam scales and dual-energy x-ray absorptiometry, respectively. RESULTS: Women randomized to the OC group tended to gain slightly less weight (adjusted mean difference (AMD) = -0.54 +/- 0.31 kg.yr, P = 0.09) and less fat (AMD = -0.35 +/- 0.25 kg.yr, P = 0.16) than those randomized to the control group. OC assignment was associated with a significant gain in lean mass relative to controls among eumenorrheic women (those who had 10 or more menstrual cycles in the year before baseline; AMD = 0.77 +/- 0.17 kg.yr, P < 0.0001) but not among women with fewer than 10 menstrual cycles in that year (AMD = 0.02 +/- 0.35 kg.yr, P = 0.96). Treatment-received analyses yielded similar results. CONCLUSION: This randomized trial confirms previous findings that OC use does not cause weight or fat mass gain, at least among young female runners. Our finding that this OC is associated with lean mass gain in eumenorrheic runners, but not in those with irregular menses, warrants examination in other studies.