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In a search for aspects of behaviour related to oral contraceptive use which might explain the favourable effect of oral contraception on the onset of rheumatoid arthritis (RA), cigarette smoking and alcohol consumption were studied as possible risk factors. Information on cigarette smoking and alcohol consumption was obtained by interview at the first visit to the outpatient clinic of 135 young women with confirmed definite or classical RA of recent onset and 378 control patients with soft tissue rheumatism or osteoarthritis. The diagnosis in all patients was confirmed by at least two years of follow up. Of the patients with RA, 44/135 (33%) were current cigarette smokers compared with 181/378 (48%) of the controls. The percentage of smokers among the controls was similar to that of women smokers in the general population. The adjusted risk of RA in women who smoked at least one cigarette a day was 0.61 (95% confidence interval (CI) 0.42 to 0.89). Thirty one (23%) patients with RA and 137 (36%) controls were alcohol drinkers. The risk of RA in women who consumed alcohol at least once a day was 0.52 (95% CI 0.33 to 0.84). The low relative risk estimates for cigarette smoking and alcohol consumption were mutually independent and also independent of oral contraceptive use, the presence of the HLA-DR4 antigen, or a positive family history of RA. The low incidence of alcohol consumption in the patients with RA might be due to the discontinuation of alcohol consumption after disease onset. The low incidence of cigarette smoking in the patients might reflect a protective effect of cigarette smoking on RA onset, possibly induced by changes in the immune system.