To quantify the influence of cigarette smoking on the risk of developing rheumatoid arthritis (RA). 679 cases and 847 controls included during May 1996-June 2000 in a case-control study, using incident cases, comprising the population aged 18-70 years of a defined area of Sweden, were investigated. A case was defined as a person from the study base who received for the first time a diagnosis of RA using the 1987 American College of Rheumatology criteria, and controls were randomly selected from the study base. Self reported smoking habits among cases and controls, and rheumatoid factor status among cases were registered. The incidence of RA in current smokers, ex-smokers, and ever-smokers, respectively, was compared with that of never-smokers. Current smokers, ex-smokers, and ever-smokers of both sexes had an increased risk for seropositive RA (for ever-smokers the odds ratio was 1.7 (95% confidence interval (95% CI) 1.2 to 2.3) for women, and 1.9 (95% CI 1.0 to 3.5) for men), but not for seronegative RA. The increased risk was only apparent among subjects who had smoked > or =20 years, was evident at an intensity of smoking of 6-9 cigarettes/day, and remained for up to 10-19 years after smoking cessation. The risk increased with increasing cumulative dose of smoking. Smokers of both sexes have an increased risk of developing seropositive, but not seronegative, RA. The increased risk occurs after a long duration, but merely a moderate intensity, of smoking and may remain for several years after smoking cessation.