One hundred-and-seven patients with juvenile rheumatoid arthritis (JRA) were studied for the presence or absence of an autoantibody in their sera directed against T cells. Using an indirect immunofluorescence technique on a fluorescence activated cell sorter, 71% of all patients were found to be positive on at least one sample. When studied according to the mode of onset of disease 75% of those with systemic onset, 70% with a pauciarticular, and 68% of those with a polyarticular onset were positive. Longitudinal studies appeared to suggest a correlation with disease activity, particularly in individual patients who were positive, while remission was almost invariably associated with negative testing for anti-T cell antibodies. These findings suggest that the anti-T cell antibody may be a useful diagnostic test in JRA and of benefit in monitoring disease activity and remission of disease.