Intracranial aneurysms are common lesions, occurring in as many as 2% of the age-corrected population of North America. Most aneurysms are small and innocuous, but some will enlarge and rupture. The clinical frequency of intact aneurysms has been increasing as a result of computed tomographic scanning and digital subtraction angiography, and this trend can be expected to accelerate in the future. Intact aneurysms are an important neurosurgical problem. Once an aneurysm ruptures, there is an associated 50 to 60% overall mortality and morbidity. In contrast, surgical correction of an unruptured aneurysm in skilled hands carries a mortality and morbidity of approximately 3%. Obviously, the optimal time for correction of aneurysm is before it ruptures. For effective management of patients with intact aneurysms, prognostic criteria for rupture are clearly needed. Aneurysm size is both theoretically and empirically a key prognostic factor for rupture. However, the critical size at which an aneurysm becomes hazardous is not known. The purpose of this investigation was to study the size of ruptured aneurysms in an attempt to infer the behavior of unruptured aneurysms. The angiographic size of aneurysms in the 1092 patients admitted to the Cooperative Aneurysm Study between 1970 and 1977 was measured. The average maximal diameter was 8.2 + 3.9 (SD) mm, and the median diameter was 7.0 mm. Seventy-one per cent of the sacs were smaller than 10 mm and 13% were less than 5 mm in diameter. Middle cerebral artery aneurysms were the largest and anterior communicating artery aneurysms were the smallest. These data suggest that aneurysms less than 1 cm in diameter are hazardous and that operation should be considered for lesions more than approximately 5 mm in diameter. Additional prognostic factors for rupture are clearly needed.