The objective was (1) to examine the prevalence of rheumatoid arthritis (RA) by a county patient register, (2) to cross-validate the register findings by a postal population survey, and (3) to estimate prevalences of disease subsets according to age, sex, and levels of physical disability. The study was performed within a county setting in the city of Oslo with 356,486 inhabitants between 20 and 79 years of age. Prevalence estimates were calculated from a county patient register comprising 1333 patients with RA and a population survey of 10,000 inhabitants. The overall prevalence of RA between 20 and 79 years was 0.437 (95% CI 0.413, 0.461) after adjusting for the incompleteness of the register by a factor of 1.17. Prevalences exceeding 1.0% was only found among females over 60 years. The prevalence of RA with MHAQ scores > or = 1.5 and > or = 2.0 (range 1-4) was 0.225 (95% CI 0.209, 0.243) and 0.099 (0.088, 0.111) respectively. We conclude that RA is less frequent in the city of Oslo than stated in most of the literature. The prevalence of RA with physical disability levels assumed to be associated with increased mortality is less than half of the overall prevalence of 0.4-0.5%.