This study describes our experience with 12 patients with white clot syndrome encountered during a recent 36-month period. The diagnosis was based on the following criteria: (1) development of thrombocytopenia of less than 100,000/mm3 during administration of heparin therapy, (2) normalization of the platelet count after an interruption in heparin therapy, (3) exclusion of other causes of thrombocytopenia, (4) a positive heparin-induced platelet aggregation test, (5) detection of white clots on pathologic examination, and (6) the presence of thrombotic complications. Of 2,500 patients who received heparin therapy, 12 (0.48%) developed white clot syndrome. Various indications, routes of administration, and types of heparin were implicated. The mean platelet nadir was 26,900/mm3, and the mean time to onset of heparin-induced thrombocytopenia was 5 days. Thrombotic complications included arterial occlusions of the legs in 11 patients, deep vein thrombosis of the legs in 9 patients (4 had pulmonary embolism), and combined arterial and venous thrombosis in 8 patients. Treatment strategies included discontinuation of heparin in all patients and intravenous infusion of dextran, followed by arterial thrombectomy in four patients, urokinase therapy in two patients for arterial complications, and insertion of Greenfield filters in six patients. All patients were given warfarin. The mortality rate was 25% and the morbidity rate was 50%. An initial platelet count should be obtained on all patients prior to receiving heparin, followed by repeat platelet counts every 2 to 3 days. Once thrombocytopenia or thrombosis is diagnosed, heparin should be discontinued and other methods of therapy considered.