Using human IgG as an antigen in an enzyme-linked immunosorbent assay (ELISA), we looked for the presence of IgM rheumatoid factor (RF) in the sera of 74 children with juvenile rheumatoid arthritis (JRA). Nine children had RF detectable by both latex agglutination and ELISA. Forty-five percent (26 of 65) of the children who were seronegative by latex agglutination were found to be positive for IgM RF by ELISA. The prevalence of IgM RF was higher in patients with polyarticular onset disease (57.4%) than in those with pauciarticular onset (38.5%) or systemic onset (27.2%) disease. The prevalence of RF was higher in sera from patients with deforming joint disease than those without deformities (P < 0.01).