Discrepant results among investigations of the association between oral contraceptive use and rheumatoid arthritis (RA) have been ascribed to shortcomings in the study methods. In the present study, we obtained detailed information on oral contraceptive use in 135 young women with confirmed definite or classic RA of recent onset and in 378 controls with soft tissue rheumatism or osteoarthritis. All patients had at least 2 years of followup to confirm the consistency of the diagnosis. Oral contraceptive use at any time before disease onset was reported by 70% of the RA patients and 85% of the controls; the latter figure corresponded to general population data for The Netherlands. This yielded a relative risk for RA of 0.39 among those who had ever used oral contraceptives and 0.58 for those using oral contraceptives at the time of symptom onset. The preventive effect of oral contraceptive use on the risk of RA was found to be independent of the dose, duration of use, or presence of HLA-DR4. The strongest protection was seen in women with a family history of RA and in women ages 31-40 at symptom onset. The conflicting results in the literature are discussed, and a possible biologic mechanism for this phenomenon is suggested.