OBJECTIVE To examine the incidence of rheumatoid arthritis (RA) in the community of Oslo, Norway; and to link the incidence to measures of disease severity. METHODS A register for RA in the county of Oslo was validated to be 85% complete for patients between 20 and 79 years of age. The incidence of RA was calculated from this register for patients with disease onset 1988-1993 (n=550). Data on health related quality of life [Modified Health Assessment Questionnaire (MHAQ), Arthritis Impact Measurement Scale 2 (AIMS2), pain, fatigue] were obtained from a postal survey (response rate 73%). RESULTS The overall annual incidence of RA over a 6 year period was 25.7/100,000 (females 36.7, males 13.8). The incidence increased with age, from 7.8 in the age group 20-29 to 61.0 between 70 and 79 years. After 5 year followup a clinically important effect on physical function (MHAQ > 1.5) was seen in 38% of the patients, on social functioning (AIMS2 social > 4) in 50%, on mental distress (AIMS2 affect > 4) in 27%, on pain (VAS > 40 mm) in 35%, and on fatigue (VAS > 40 mm) in 41%. CONCLUSIONS The incidence of RA was 25.7/100,000 in the population of Oslo. After 5 years, 40-50% had clinically important changes in health status. The annual disease incidence with clinically important effect on physical function after 5 years was 10/100,000 individuals at risk.