Ten of 12 patients with systemic lupus erythematosus (SLE) who had hemolytic anemia and thrombocytopenic purpura (Evans' syndrome) during their course had evidence of antiphospholipid antibodies either because they had a false positive VDRL test (8 patients), a prolonged partial thromboplastin time (5 patients), a lupus anticoagulant (3/4 patients), and/or anticardiolipin antibodies as determined by an ELISA method (7 patients). Antibodies to cardiolipin were found in very high levels (up to 38 standard deviations above the mean of normal controls) and were of both IgG and IgM isotypes. The 2 patients with SLE and Evans' syndrome who did not have evidence of antiphospholipid antibodies were studied at the onset of SLE which occurred with Evans' syndrome. Although cardiolipin is not a constituent of the cell wall of either platelets or erythrocytes, other phospholipids that cross react antigenically with cardiolipin are and can be exposed through cell damage. This could be a mechanism whereby hemolytic anemia and thrombocytopenia could occur in the same patient with SLE. Whether absorption of the antiphospholipid antibody during the acute episode of hemocytopenia could occur, and thus prevent its detection at such time, remains undetermined.