The aim of this study was to compare the effects of intense endurance vs strengthening exercise on bone mass and serum testosterone levels in male athletes. Bone mineral density (BMD) of the total body and spine and serum testosterone levels were measured in male rowers (n = 12), triathletes (n = 8) and sedentary controls (n = 13). The total body scan also gave values for percentage body fat and regional bone densities. Calcium intake and physical activity levels were measured by questionnaire. The rowers had significantly higher BMD in the spine and total body than the triathletes (P < 0.01 and P < 0.05 respectively) and sedentary controls (P < 0.01 and P < 0.05). There were no differences between the triathletes and controls. Serum testosterone levels were significantly lower in the triathletes than in the controls (P < 0.05); there was no significant difference between the rowers and controls. All groups fell within the normal range for testosterone. In a step-wise multiple regression, including age, body mass, height, calcium intake and activity, no single factor had a significant effect on spine BMD. Body mass had a significant effect on total body BMD and could account for the differences between the groups. A significant positive correlation was found between calcium intake and total body BMD. The heavy weight training typical of rowing training seemed to result in significant bone accretion. The low testosterone levels in the triathletes may have negated any positive effect of the increased exercise on BMD.