Risk factors for rheumatoid arthritis were examined in a cohort of 121,700 female nurses aged 30-55, followed in the Nurses' Health Study. Baseline information on reproductive variables, cigarette smoking, obesity, and other variables was obtained in 1976 and updated every 2 years. Cases of rheumatoid arthritis were defined by standardized questionnaire and review of medical records. During 883,187 person-years of follow-up, 217 new cases of rheumatoid arthritis were identified (115 had definite rheumatoid arthritis and 102 had "undifferentiated polyarthritis"). When compared with women who experienced menarche at age 13 years, the age-adjusted relative risk of rheumatoid arthritis among women with early menarche was 1.9 (95% confidence interval, 0.9-2.4). There were, however, no significant associations between parity, age at birth of the first child, menopause, or obesity and the incidence of rheumatoid arthritis. Cigarette smokers had a slight apparent increase in the risk of rheumatoid arthritis. Among current smokers, the age-adjusted relative risk for RA rheumatoid arthritis was 1.3 (0.9-2.1); among former smokers, the relative risk was 1.5 (0.9-2.3).