Clinical and immunogenetic factors were compared in 214 patients with sporadic rheumatoid arthritis (RA) and 117 patients from 52 multiplex families. Sex distribution, articular disease severity and seropositivity for rheumatoid and antinuclear factors were similar in familial and sporadic disease. There was a trend for Sjögren's and Felty's syndromes to be more frequent in familial RA but extraarticular disease features were otherwise similar in the 2 RA disease groups. Mean age of onset was 41.1 years in familial and 46.5 years in sporadic RA (p less than 0.0006); 67% of family probands, 74% of affected relatives and 57% of sporadic patients were HLA-DR4 positive (p less than 0.05 affected relatives vs sporadic). The similarity of clinical features found in familial and sporadic RA justifies the use of families with RA to study aspects of disease pathogenesis.