We studied whether several modifiable factors were associated with the risk of total hip replacement due to hip osteoarthritis among women.We identified 568 women from the Nurses' Health Study who reported total hip replacement due to primary hip osteoarthritis on questionnaires from 1990 to 1996, using a validated algorithm. The relation of potential risk factors, such as age, body mass index, physical activity, smoking, alcohol intake, and hormone use, to hip replacement was assessed using pooled logistic regression models. Higher body mass index was associated with an increased risk of hip replacement due to osteoarthritis (P for trend = 0.0001). Compared with women in the lowest category of body mass index (<22 kg/m(2)), those in the highest category of body mass index (> or =35 kg/m(2)) had a twofold increased risk (95% confidence interval [CI]: 1.4 to 2.8), whereas those in the highest category of body mass index at age 18 years had more than a fivefold increased risk (95% CI: 2.5 to 10.7). Age also had a positive association; women aged > or =70 years were nine times more likely to have hip replacement than those aged <55 years (95% CI: 5.4 to 13.9). Recreational physical activity, smoking, alcohol use, and postmenopausal hormone use were not associated with an increased risk of hip replacement. In the Nurses' Health Study, higher body mass index and older age significantly increased the risk of total hip replacement due to osteoarthritis. Part of this risk appeared to be established early in life.