We studied 500 consecutive patients with systemic lupus erythematosus (SLE) for antibodies to phospholipids (APLA) by an ELISA method using cardiolipin as antigen and antiimmunoglobulins G, M and A to determine their isotype. Once entered into this prospective study the patients were followed for up to 16 months (mean 7.7 +/- 4.72 SD) with periodic determinations of APLA. Of the 500 patients with SLE, 88 had had thrombocytopenia, 25 had had hemolytic anemia, 25 had had both, and 362 had no history of these hemocytopenias. If we considered the odds ratio of these 362 patients for having high titer APLA as 1, patients with a history of thrombocytopenia, hemolytic anemia or both had significantly higher odds ratios of having APLA than did those without hemocytopenia. Patients with thrombocytopenia had significantly higher levels of IgG APLA, those with hemolytic anemia had significantly higher titers of IgM APLA and patients with both had significantly higher titers of both of these APLA isotypes, than did patients without hemocytopenias. A correlation between positive direct Coombs' tests and IgM APLA was also found. We conclude that APLA is associated with these hemocytopenias in SLE. This might be due to their interaction with negatively charged phospholipids in the cell walls of the respective cells.