We studied 137 patients who were treated with heparin for cerebral infarction (73), partially reversible ischemic neurologic deficit (22), or transient ischemic attack (42). Platelet counts were performed before therapy, twice weekly, and at cessation of therapy. Platelets decreased in 118 patients (86%). In 21 (15.3%), platelets dropped ≥ 40%; 9 of 14 new ischemic events and three of six deaths occurred in this group of patients. Because there was a significant association between poor outcome and platelet drop ≥ 40% (p < 0.001), we believe that platelets should be monitored frequently when patients are treated with heparin for ischemic cerebrovascular disease.