Synovial fluid (SF) lymphocytes from 4 patients with pauciarticular juvenile rheumatoid arthritis (JRA) and 4 patients with polyarticular JRA were examined for their phenotypic and functional characteristics. In all 8 patients there was a high proportion of activated SF T cells, together with an increased proportion of CD2+CD3- and the presence of CD3+CD4-CD8-WT31- lymphocytes. The functional analysis at the clonal level in 5 patients (427 clones) showed a relevant proportion of cytotoxic T cell clones, which were not confined to typically cytolytic phenotypes, but were also present among CD3+CD4+CD8- cultures. Compared to those with pauciarticular JRA, patients with polyarticular disease had a significantly higher proportion of T cell clones with cytotoxic activity. Although derived from a limited number of patients, our data suggest a direct involvement of T cells in the pathogenetic mechanisms that originate and maintain the articular damage, and the possibility of different or more pronounced T cell reactivities in the clinically more diffuse JRA types.