Thirty-four of 87 consecutive patients with subarachnoid hemorrhage from a cerebral aneurysm had a premonitory minor leak. There were 12 men and 22 women, aged 25 to 73 years (mean 44.4 years). Twenty-two had a small and 12 had a large aneurysm located on the internal carotid artery (17 cases), anterior communicating artery (10 cases), middle cerebral artery (five cases), and pericallosal artery (two cases). Fifty-two percent of patients with a minor leak from an internal carotid artery aneurysm had ipsilateral, hemicranial, hemifacial, or periorbital pain. Half of the patients initially saw a physician, but in no case was the correct diagnosis made. Twenty-five patients had a major rupture within 24 hours to 4 weeks after findings suggesting a minor leak, with a mortality rate of 53%. Nine other patients were diagnosed by lumbar puncture or computerized tomography (CT) scanning after initial misdiagnosis and were operated on, without mortality, before a major rupture could occur. The CT scans were negative in 55% of patients with a minor leak, but lumbar puncture, when performed, was always positive. A minor leak prior to major aneurysmal rupture is a common occurrence and, if unrecognized, is associated with a high mortality. Computerized tomography scanning is unreliable in diagnosing this event, and lumbar puncture is the examination of choice once intracranial hypertension has been ruled out.