Twenty-two patients developed significant thrombocytopenia (5,000 to 96,000/cu mm; mean, 29,000/cu mm) while receiving prophylactic or therapeutic heparin. Seventeen of them developed serious thrombohemorrhagic complications which accelerated the deaths of six and contributed to the late death of one. Cessation of heparin therapy led to an immediate remission of the thrombohemorrhagic complications and thrombocytopenia, with no patient who was not already moribund dying, once appropriate therapy had been instituted. Platelet-count monitoring is recommended for all patients receiving heparin for more than 6 days, with cessation of heparin therapy mandatory for the successful management of patients with this disorder. Evidence is presented for an immunologic etiology for this disorder.