Three cases of reactive arthritis associated with Yersinia enterocolitica bowel infection are reported. In one case Y. enterocolitica was cultured from stools while all three exhibited a significant increase in serum antibody titres to Y. enterocolitica. These are believed to be the first reports of Yersinia reactive arthritis in Australia. Synovial membrane biopsy in one case revealed a mixed inflammatory cell infiltrate the appearance of which was quite dissimilar to that of rheumatoid arthritis (RA). Mononuclear cells of the peripheral blood, but not the synovial fluid from another patient had reduced functional activity compared to RA patients as determined by response to phytohemagglutinin stimulation and allogeneic responses. Large numbers of HLA-DR and acid phosphatase positive macrophages were also found in the synovial fluid of this patient. The reason for joint involvement in Y. enterocolitica reactive arthritis is unknown and further work is necessary.