Thirteen of the 14 patients had been discharged from the hospital in satisfactory condition after receiving heparin therapy. Within a few weeks, they developed symptoms and returned to the hospital. Test results showed blood clots in the veins of 12 patients and in the arteries of 4 patients (2 patients had clots in both types of blood vessels). Doctors gave heparin to 11 patients to treat the blood clots because in each case, the doctors did not realize that the patient had heparin-induced thrombocytopenia. After receiving heparin therapy, the number of platelets decreased dramatically in all patients. When the doctors found antibodies to heparin, they made the correct diagnosis, stopped the heparin therapy, and started other treatments.