A 10-year-old boy with confirmed congenital agammaglobulinaemia presented with polyarthritis while on gammaglobulin replacement therapy. Initial cultures of material aspirated from an abscess and of joint fluid were negative, and symptoms progressed despite antibiotic therapy. Synovial-biopsy material, cultured specifically for mycoplasmas, was positive for Ureaplasma urealyticum as were the blood, abscess fluids, throat-swab, and nasopharyngeal secretions. Therapy, based on in-vitro studies of antibiotic susceptibilities of the organism, resulted in the eradication of the infection and resolution of the arthritis. These findings suggest that U. urealyticum may be capable of inducing polyarthritis in man.