Running is a popular sport, but in some studies long distance running in women has been related to reduced bone mass and a potential risk of osteoporosis. To investigate the impact of running on bone mass in men, 120 healthy, physically active men (19-56 yr old; running 0-160 km/week) were studied. Bone mineral content was measured in the lumbar spine, total body, and proximal femurs by dual energy x-ray absorptiometry and in the forearm by single photon absorptiometry. Bone turnover was assessed by urinary pyridinium cross-links, plasma osteocalcin, and serum alkaline phosphatase. Lumbar bone mineral content was negatively correlated to the weekly distance run (r = -0.37; P < 0.0001), with a difference of 19 +/- 5% (mean +/- SEM) between controls and elite runners. A similar relation was found for all measurement sites. After adjustment for possible confounders, the correlations remained statistically significant in areas with a high proportion of trabecular bone. Bone turnover parameters were 20-30% higher in the elite runners, whereas sex hormone status was unrelated to running activity. We conclude that male long distance runners had reduced bone mass and increased bone turnover compared to controls, which suggests accelerated bone loss. The pathophysiological explanation was not clear, but sex hormones did not seem to play a key role.