The significance of thrombocytopenia in systemic lupus erythematosus (SLE) is unclear. Some researchers have found it associated with severe disease, others with mild disease. Thrombocytopenia (platelets less than 100,000) occurred in 21 patients seen at an SLE clinic over 18 months. Prospective assessment of 19 (non-SLE causes excluded) revealed 2 distinct subgroups. Seven were thrombocytopenic only during severe multisystem flares. Twelve had chronic thrombocytopenia with intermittent mild flares in other systems. Serious bleeding was rare in both subgroups. It was concluded that thrombocytopenia clearly is not a prognostic indicator.