We describe six boys with homozygous sickle cell disease, aged 7 to 13 years, in whom acute, severe neurologic abnormalities developed 1 to 11 days after partial exchange transfusion was performed to treat priapism that was unresponsive to more conservative therapy. Hemoglobin levels were 10.5 to 13.4 gm/dl (mean 12.1 gm/dl), and hemoglobin S levels were 18% to 33% (mean 27%) before the onset of neurologic complications. Severe headache was the initial finding in five patients, four of whom had increased intracranial pressure and three of whom required tracheal intubation and hyperventilation. Four patients had seizures; three had focal neurologic deficits for more than 24 hours. Cerebral arteriography demonstrated vascular abnormalities, including irregularity, stenosis, and complete occlusion of vessels. Patients treated with regular erythrocyte transfusions had no recurrence of neurologic signs or symptoms when hemoglobin S levels were kept at 30% to 50%. The occurrence of serious neurologic complications after partial exchange transfusion in patients with homozygous sickle cell disease from three centers indicates the possibility of a causal relationship between the events. Early and thorough investigation of neurologic symptoms, especially severe headache, is warranted in this clinical setting.