Forty-three patients with aneurysmal subarachnoid hemorrhage entered a nimodipine trial in the Department of Neurosurgery, Yonsei university to determine the efficacy of the drug in preventing vasospasm and to evaluate the tolerability of this calcium channel blocker. Thirty-three patients completed the study. Treatment was started within four days of initial bleeding and continued for two weeks. Delayed neurological deficits developed in seven of the 33 patients-four from vasospasm, two from elevated intracranial pressure, and one from recurrent bleeding. The incidence of symptomatic vasospasm which developed after calcium channel blocker (nimodipine) treatment was 12.1%, which is about one third of the rate experienced at our department during the past five years (33.2%). Twenty-five patients were operated on without surgical mortality and the morbidity rate was 8%. Side effects due to nimodipine treatment were reversible and insignificant. This study suggests that treatment with a calcium channel blocker that has a selective cerebrovascular effect may prevent or reduce the incidence of delayed ischemic deficits in patients with aneurysmal subarachnoid hemorrhage.