Bone density in 13 male long distance runners (28.7 +/- 1.2 yr, 67.6 +/- 2.0 kg) and 11 male nonrunners (26.8 +/- 1.3 yr, 71.0 +/- 2.2 kg) was compared. Bone was measured at the lumbar spine and mid-tibia using dual photon absorptiometry and at the mid-radius using single photon absorptiometry. Runners (mean weekly training 92.2 +/- 6.3 km) had significantly lower (P less than 0.05) vertebral bone mineral density (1.12 +/- 0.03 g.cm-2) than nonrunners (1.24 +/- 0.04 g.cm-2). Tibial and radial bone mineral density did not differ between the groups. Daily calcium intake for runners (1,373 +/- 486 mg) and nonrunners (1,267 +/- 236 mg) exceeded the RDA. The results of this study suggest that long distance running may lead to decreased vertebral bone mineral density. The hormonal changes that occur with endurance training may contribute to this decrease.