Seven pediatric patients with monoarticular arthritis, three of whom had a recent onset form and the remaining four a disease of longer duration, were examined for possible modifications of their immunological parameters. The diagnosis of JRA was made on all these patients according to the ARA criteria after a follow-up of at least two years. Humoral and cellular abnormalities of the immune system were searched for in peripheral blood, synovial fluid and synovial membrane. No evidence for complement consumption and for increased levels of immune-complexes was found in the sera and in the synovial fluids of these patients, who were all seronegative. Some patients had antinuclear antibodies in their sera and synovial fluids. With regard to the lymphocyte distribution, whereas only some patients had an increased number of circulating B cells, the majority had a decreased CD4+/CD8+ ratio in the synovial fluid compared to the ratio found in the peripheral blood. A massive infiltration of CD4+ cells and macrophages and the presence of a substantial number of OKT9+ cells was found in the synovial membranes.