Two years after jejunal-ileal bypass surgery for obesity, a 25-year-old man developed intravascular hemolysis, thrombocytopenia, and neutropenia. The patient's erythrocytes were coated with complement components (C4/C3) and his serum induced complement-dependent immune lysis of chromium-51-labeled platelets. Serum [125I]-C1q binding activity (a measure of the presence of immune complexes) was increased, and serum C4 and C3 hemolytic titers were depressed. Immune complex-mediated complement activation apparently accounted for the blood cell destruction in this patient.