The frequency and causes of diagnostic errors were studied in patients with intracranial tumors. Of 34 tumors found unexpectedly at necropsy, 12 were considered purely incidental, while 22 had caused neurologic disturbances and had been erroneously diagnosed during life. Confusion with cerebrovascular disease was most common, particularly in older patients. Glioblastoma multiforme and meningioma were the most frequent types of tumor in the series, and each presented illnesses simulating cerebrovascular thrombosis, carotid artery insufficiency, and subarachnoid hemorrhage. In 17 cases, including 7 meningiomas, there was, in retrospect, sufficient clinical or laboratory evidence to cause considerable doubt regarding the clinical diagnosis. Most of the tumors would have been recognized if contrast intracranial studies had been performed.