• We tested the hypothesis that intracranial aneurysm develops because of viral infection that produces arterial damage, and that aneurysmal rupture is related to viral infection. The following viral agents were studied: influenza A, influenza B, and respiratory syncytial viral titers, by the indirect immunofluorescence method, in 29 patients and 29 controls; herpes simplex virus titers, by immunofluorescence, in 31 patients and 31 controls; rubella viral titers (Rubazyme test), in 34 patients and 34 controls; and influenza A and B viral titers, by the complement fixation method, in 54 patients and 54 controls. Patients were selected on the basis of documented aneurysmal subarachnoid hemorrhage. Analysis, with derivation of the ϰ2 method, to compare titer levels in patients and controls for each specific viral titer did not reveal positive correlations between the viral titers and aneurysmal subarachnoid hemorrhage.