Context: Physical activity can reduce the risk of hip fractures in older women, although the required type and duration of activity have not been determined. Walking is the most common activity among older adults, and evidence suggests that it can increase femoral bone density and reduce fracture risk. Objective: To assess the relationship of walking, leisure-time activity, and risk of hip fracture among postmenopausal women. Design, Setting, and Participants: Prospective analysis begun in 1986 with 12 years of follow-up in the Nurses' Health Study cohort of registered nurses within 11 US states. A total of 61 200 postmenopausal women (aged 40-77 years and 98% white) without diagnosis of cancer, heart disease, stroke, or osteoporosis at baseline. Main Outcome Measures: Incident hip fracture resulting from low or moderate trauma, analyzed by intensity and duration of leisure-time activity and by time spent walking, sitting, and standing, measured at baseline and updated throughout follow-up. Results: From 1986 to 1998, 415 incident hip fracture cases were identified. After controlling for age, body mass index, use of postmenopausal hormones, smoking, and dietary intakes in proportional hazards models, risk of hip fracture was lowered by 6% (95% confidence interval [CI], 4%-9%; P<.001) for each increase of 3 metabolic equivalent (MET)-hours per week of activity (equivalent to 1 h/wk of walking at an average pace). Active women with at least 24 MET-h/wk had a 55% lower risk of hip fracture (relative risk [RR], 0.45; 95% CI, 0.32-0.63) compared with sedentary women with less than 3 MET-h/wk. Even women with a lower risk of hip fracture due to higher body weight experienced a further reduction in risk with higher levels of activity. Risk of hip fracture decreased linearly with increasing level of activity among women not taking postmenopausal hormones (P<.001), but not among women taking hormones (P=.24). Among women who did no other exercise, walking for at least 4 h/wk was associated with a 41% lower risk of hip fracture (RR, 0.59; 95% CI, 0.37-0.94) compared with less than 1 h/wk. More time spent standing was also independently associated with lower risks. Conclusion: Moderate levels of activity, including walking, are associated with substantially lower risk of hip fracture in postmenopausal women.