Abstract
A case is reported of primary late-onset hypogammaglobulinaemia associated with a chronic seronegative, nonerosive arthritis, which was complicated by an episode of septic arthritis due to Mycoplasma pneumoniae. The patient had subcutaneous nodules which have not been regarded previously as a feature of the chronic arthritis associated with hypogammaglobulinaemia. The diagnosis of septic arthritis was delayed for 2 months until synovial fluid was specifically cultured for mycoplasmas. This delay resulted in considerable joint destruction. The importance of searching for mycoplasmas in similar cases is emphasised.