This is a two-year randomized trial of the effects of oral contraceptives on bone mass and stress fracture incidence among 150 female competitive distance runners in the age range 18-25 years. The Coordinating Center is at Stanford University and bone mass in being measured at five sites: Massachusetts General Hospital, University of California Los Angeles, University of Michigan, Stanford University/Palo Alto VA Medical Center, and Helen Hayes Hospital in West Haverstraw NY. Athletes are currently being recruited from the areas around these five clinical sites. In August 2001 we received IRB approval to expand our recruitment in the Los Angeles area to non-student runners, and this expansion has considerably increased our pool of eligible runners. To date, over all five clinical sites, 101 have been randomized and an additional 45 successfully screened and being scheduled for their bone densitometry. We expect to complete baseline examinations and randomization in early 2002. Final results will thus not be available until early 2004.