This is the first reported synovial biopsy of a patient with concurrent onset of cutaneous T cell lymphoma and polyarthritis. The biopsy revealed a nonspecific chronic inflammatory process. Immunoperoxidase technique using monoclonal antibodies showed a mixture of helper T cells, suppressor T cells and B cells in the synovial infiltrate. Tumor infiltration was not seen. Our findings indicate that the arthritis associated with cutaneous T cell lymphoma is most likely a reactive phenomenon.