In general, physical exercise appears to have favorable effects on the skeleton. However, a few recent reports have described negative effects, including reduced bone density (BMD) and high bone turnover in runners. The aim of our study was to compare endurance runners to controls with respect to BMD at different sites and ultrasound transmission through the peripheral skeleton, and to use PTH, total serum calcium, and biochemical markers of bone metabolism as a complement in evaluating the effects of endurance running on bone. Thirty runners (mean age 32 years, range 19-54 years) participated in the study. Their main form of training consisted of endurance running at moderate intensity for about 7 hours (range 2-12 hours) per week, and they had been active in their sport for about 12 years (range 1-21 years). For a comparison, 30 age- and sex-matched population based controls were investigated. BMD values, measured by dual energy X-ray absorptiometry (DXA), were higher in runners than in controls for the total body (3.6%; P = 0.03), legs (9.6%; P = 0. 001), femoral neck (10.0%; P = 0.01), trochanter (9.9%; P = 0.01), and Wards triangle (11.8%; P = 0.02), but not in the lumbar spine or in the forearm measured by single energy X-ray absorptiometry (SXA). The quantitative ultrasound measurement of the calcaneus also revealed higher values in runners than in controls for both broadband ultrasound attenuation (9.2%; P = 0.002) and speed of sound (3.1%; P = 0.0001). At all sites, BMD was related to ultrasound measurements in controls, but no such relationship was evident in runners. Concentrations of parathyroid hormone (PTH) were lower (23.2%; P = 0.02) in runners than in controls, whereas total serum calcium concentrations were slightly higher (3.0%; P = 0.003). The levels of PICP (bone formation) and ICTP (bone resorption) in serum were lower (18.0%; P = 0.03 and 22.2%; P = 0.004, respectively) in runners than in controls, but no differences were seen for osteocalcin or bone specific alkaline phosphatase (b-ALP). In conclusion, BMD at the focus of strain for running, that is, the legs, is higher in endurance runners when compared to matched controls. Low bone turnover in runners, indicated by lower levels of PTH and biochemical markers of bone metabolism, point to an influence of endurance running at the cellular level.