The purpose of this study was to determine total and regional bone mineral density (BMD) in highly competitive young adult and master male cyclists. Three groups of men were studied: older cyclists (51.2+/-5.3 years, n=27); young adult cyclists (31.7+/-3.5 years, n=16); and 24 non-athletes matched by age (+/-2 years) and body weight (+/-2 kg) to the master cyclists. All of the master cyclists had been training and racing for a minimum of 10 years (mean 20.2+/-8.4 years) and engaging in little to no weight-bearing exercise. The younger cyclists also engaged in little weight-bearing exercise and had been training and racing for 10.9+/-3.2 years. Age-matched controls were normally active. The History of Leisure Activity Questionnaire was used to determine the influence on BMD of self-reported total and weight-bearing exercise during three periods of life: 12-18 years, 19-34 years, and 35-49 years. BMD (measured by DXA) of the spine (L2-L4) and total hip was significantly (P<0.033) lower in the master cyclists compared to both age-matched controls and young adult cyclists. Total body BMD was lower in the master cyclists compared to the young-adults (P<0.033). Furthermore, four (15%) of the master cyclists, but none of the men in the other groups, had T-scores (spine and/or hip) lower than -2.5. Weight-bearing exercise performed during teen and young adult years did not appear to influence BMD, as there were no differences at any site between those within the upper and lower 50th percentiles for weight-bearing exercise during the 12-18, 19-34, or 35-49 year time periods. These data indicate that master cyclists with a long history of training exclusively in cycling have low BMD compared to their age-matched peers. Although highly trained and physically fit, these athletes may be at high risk for developing osteoporosis with advancing age.