The effect of long-distance running on bone mass was assessed in 10 premenopausal and 9 estrogen-deprived postmenopausal women and compared to that in closely matched sedentary control women. Vertebral trabecular bone density (VBD) was determined by computed tomography and radial cortical bone density (CBD) by single-photon absorptiometry. Physical fitness was assessed by determining maximal oxygen consumption (VO2max) on a treadmill run to exhaustion. VBD was 183 + 7 mg/cm3 and VO2max was 48 + 1 ml/kg per minute in young women runners and 163 + 8 mg/cm3 and 32 + 2 ml/kg per minute in sedentary young women. A positive correlation was noted between VBD and VO2max in these groups (r = 0.509, p < 0.03). Despite a significantly higher VO2max in postmenopausal women runners compared with sedentary controls (37 + 2 versus 24 + 2 ml/kg per minute), VBD was identical (112 + 5 versus 111 + 5 mg/cm3) and no correlation was seen between VBD and VO2max (r = 0.187, p = 0.457). Radial cortical bone density was not different between the runners or sedentary groups in young women (0.738 + 0.01 versus 0.732 + 0.1 g/cm2) or postmenopausal women (0.617 + 0.3 versus 0.665 + 0.4 g/cm2). These results suggest that although physical fitness enhances vertebral bone density in premenopausal women, it does not appear to prevent age-and/or sex steroid deficiency-induced bone loss in postmenopausal women.