['To analyse the relationship between different rheumatoid factor (RF) isotype patterns and the prevalence of RA.', 'Serum samples, collected between 1973 and 1983 from nearly 14,000 randomly selected individuals, were screened for elevation of RF. In 1987, 173 RF positive and 156 matched RF negative participants were evaluated clinically.', 'Participants with elevation of only one RF isotype, most commonly IgM, did not have significantly higher prevalence of RA than the RF negative controls. Of the 17 RF positive individuals who were diagnosed with RA, 14 (82%) had a combined elevation of IgM and IgA RF.'] Is elevation of only one rheumatoid factor isotype associated with increased prevalence of rheumatoid arthritis -- a population based study?