A total of 76 women were enrolled and 55 naturally postmenopausal women completed a 12 month study investigating the effects of aerobic training plus calcium supplementation on lumbar (L2-4) bone mineral density (BMD) and forearm BMD. Training was conducted on treadmills at 70-85% of maximal heart rate for 30 or 45 minutes, three times per week for 12 months. Lumbar BMD was measured with dual-photon absorptiometry and forearm BMD with single-photon absorptiometry. Groups were similar with respect to age, weight, months since menopause, height, maximal oxygen uptake (VO2max), and lumbar and forearm BMD upon entering the study. Following training, percentage changes in VO2max were significantly different between the control and exercise groups but not between exercise groups. ANOVA evaluating lumbar BMD revealed a nonsignificant interaction effect and no significant changes (p > 0.05) between groups or times. The control (N = 19), 30 minute (N = 20), and 45 minute (N = 16) groups percentage lumbar BMD changes (X +/- SD) over 12 months were -0.61 +/- 3.40, -0.48 +/- 3.63, and 0.81 +/- 4.53%, respectively. The 95% confidence limits for percentage changes in lumbar BMD for the control, 30 minute, and 45 minute groups were -2.25 to 1.02, -2.18 to 1.22, and -1.16 to 3.22%, respectively. Forearm BMD changes were also not significant. Improvement in lumbar BMD was weakly but positively correlated with improvements in VO2max, r = 0.28, p < 0.05. Women < or = 6 years of the onset of menopause had an accelerated lumbar BMD loss compared to subjects who were > 6 years postmenopausal, and this subset's BMD changes were examined.(ABSTRACT TRUNCATED AT 250 WORDS)