There is some evidence that sex hormones may influence the incidence of rheumatoid arthritis. Obesity has been found to increase endogenous estrogens, particularly among postmenopausal women. Although the association of endogenous hormones with smoking and alcohol use is less clear, we examined the relation of these three factors with rheumatoid arthritis in women. We conducted a population-based case-control study of rheumatoid arthritis in King County, WA, and Group Health Cooperative of Puget Sound. We compared 349 incident cases of rheumatoid arthritis with 1,457 controls selected by random digit dialing or by random selection from the enrollment files of Group Health Cooperative. We conducted personal interviews to collect data on weight, smoking, alcohol history, reproductive variables, and other demographic variables. Women with 20 or more pack-years of smoking had a relative risk of 1.5 [95% confidence interval (CI) = 1.0-2.0] compared with never-smokers. Post-menopausal women who averaged more than 14 alcoholic drinks per week had a reduced risk of rheumatoid arthritis (relative risk = 0.5; 95% CI = 0.2-1.7). Women in the highest quartile of body mass index had a risk of 1.4 (95% CI = 1.0-2.0) relative to women with lowest body mass index.