['To determine the risk of rheumatoid arthritis (RA) in first degree relatives of a true population based sample of probands with inflammatory polyarthritis.', 'In a case-control study, a two stage screening procedure was used to ascertain the prevalence of RA in 518 first degree relatives of 207 Norfolk Arthritis Register cases registered in 1990 and 414 first degree relatives of 180 local controls. An initial joint symptom and medical history questionnaire was followed by a physical examination, and serological and radiological evaluation of those with symptoms.', 'The prevalence of RA in the first degree relatives of all the Norfolk Arthritis Register cases was 7.7/1000, compared with 4.8/1000 in the first degree relatives of the controls, with a risk ratio of 1.6 (95% confidence interval 0.3 to 8.7). This very modest increase was also seen when the analysis was restricted to the first degree relatives of Norfolk Arthritis Register cases who satisfied the American Rheumatism Association criteria for RA: prevalence rate 7.2/1000.'] Is occurrence of rheumatoid arthritis increased in the first degree relatives of a population based inception cohort of inflammatory polyarthritis?