High-dose prostacyclin treatment during cardiopulmonary bypass reduces platelet activation and possibly postoperative blood loss. A side-effect is arterial hypotension. We studied the incidence of cerebral complications in 79 patients requiring coronary bypass. Only patients without known cerebrovascular disease were studied. Thirty-nine patients received prostacyclin 50 ng/kg/min during cardiopulmonary bypass and 40 patients served as controls. Mean arterial blood pressure in the group given prostacyclin was below 30 mm Hg during the first 30 minutes of bypass but remained above 60 mm Hg in the control group. Postoperative neurological examination revealed transient cerebral dysfunction in six control patients and two prostacyclin-treated patients. Investigation of cerebrospinal fluid showed signs of blood-brain barrier damage in 12 control and seven prostacyclin-treated patients. Cytologic changes in cerebrospinal fluid consistent with brain tissue damage occurred in two control patients but in no patient given prostacyclin. Myelin basic protein and adenylate kinase in cerebrospinal fluid were assayed as being markers of brain damage. Myelin basic protein was within the normal range in all patients. Adenylate kinase was moderately increased (greater than 0.035 U/L) in five of 15 control patients and six of 13 prostacyclin-treated patients. We conclude that treatment with prostacyclin 50 ng/kg/min during cardiopulmonary bypass does not increase the risk of postoperative cerebral damage.