Anticardiolipin antibody levels were determined in 116 patients with systemic lupus erythematosus and related autoimmune disorders. Forty-three of these patients had a history of thrombocytopenia--36 of whom had SLE, three primary Sjögren's syndrome, two rheumatoid arthritis and two mixed connective tissue disease. IgG anticardiolipin antibody levels were raised in 31 (72%) of the 43 patients and IgM anticardiolipin antibody levels were raised in 19 (44%). There was a strong statistical correlation between thrombocytopenia and raised anticardiolipin antibody levels of both the IgG (P less than 0.001) and IgM (P less than 0.01) immunoglobulin classes. Of the 20 patients with the highest IgG anticardiolipin antibody levels 16 had a history of thrombocytopenia. We suggest that anticardiolipin antibodies may play a direct role in mediating platelet destruction in autoimmune disorders.