PURPOSE: Hormonal changes associated with menopause, chronological aging, and lifestyle, specifically physical activity, may all influence the changes in body composition and fat distribution experienced by midlife women. This cross-sectional study examined those relations in a representative sample of 248 white and Chinese women, ages 47-57, participating in an ancillary study to the Study of Women’s Health Across the Nation (SWAN), a multi-center, longitudinal investigation of the natural history of the menopause in a racially/ethnically diverse cohort. METHODS: Body composition (lean mass, percent body fat) was assessed with dual energy x-ray absorptiometry, and central adiposity was determined by waist circumference. Physical activity was assessed from 7 d of accelerometer recordings. Menopausal status was based on self-reported bleeding patterns. RESULTS: Higher levels of physical activity, particularly vigorous-intensity activity, were generally independently associated with decreased percent body fat and smaller waist circumference, although these findings were not statistically significant in the Chinese women. Among the white women, every half a standard deviation increase in total activity was associated with a 1.6-point decrease in percent body fat (P = 0.002). Waist circumference decreased from 96.2 cm (SE = 1.04) in those doing no vigorous-intensity activity to 81.4 cm (SE = 1.05) in those doing 10 min or more a day (P for trend = 0.05). For both the whites and the Chinese, late peri- and postmenopausal status was associated with lower lean mass, and among the Chinese, tended to be associated with higher percent body fat. CONCLUSION: These findings suggest that regular physical activity may help to mitigate the tendency for weight gain and adverse changes in body composition and fat distribution that accompany aging and the menopausal transition.