The clinical course of children with IgM rheumatoid-factor-positive chronic arthritis closely resembles that of seropositive rheumatoid disease in adults. The frequency of HLA DR4 is known to be increased in adults with seropositive rheumatoid arthritis, but the first major report on childhood arthritis did not suggest a correlation, though only 8 seropositive cases were included. Fifty-two children with polyarthritis beginning before the age of 16 who persistently carried IgM rheumatoid factor were studied. HLA DR4 was present in 60% of these children but was found in only 29% of 93 patients with seronegative juvenile arthritis and 27% of a normal adult population. These results suggest immunogenetic similarities in patients with seropositive arthritis irrespective of age of onset.