Previously, only three studies with representative samples of patients with ruptured intracranial aneurysms have provided detailed results of prospective, repeated, neuropsychological assessments after surgery. These studies apparently disagree with regard to occurrence of cognitive deficits and to degree of improvement between early and delayed follow-ups. The present paper attempts to analyze the conditions underlying these differences in results. As a first step in this analysis we present a comprehensive, prospective, neuropsychological investigation of a consecutive sample of 41 patients with rupture of a supratentorial aneurysm, assessed 4 and 12 months after surgery. It is concluded that a prorated course of improvement of a wide specter of psychological functions may be revealed, but that sensitive tests and large samples are needed to establish the range of deficits and improvements with time. Differences in patient selection with respect to severity of the acute clinical state and delayed deterioration apparently contribute importantly to the discrepance in previously reported outcome.