The incidence of hydrocephalus and vasospasm and the relationship between them were analyzed retrospectively in 87 patients with subarachnoid hemorrhage from ruptured intracranial aneurysms. Sixty-seven per cent of the patients showed ventricular enlargement on a computed tomographic scan done within 30 days of the hemorrhage; in patients whose first scan was done within 3 days of the hemorrhage, 63% seemed to have ventricular enlargement by a neuroradiologist's interpretation. Shunts were required in 14% of the patients because of delayed neurological deterioration or enlarging ventricles; 3% required ventriculostomy shortly after admission. Seventy-four per cent of the patients had angiographic spasm on an angiogram done within the first 30 days after hemorrhage. Sixty-two per cent of the patients had both hydrocephalus and vasospasm: 22% had neither. Five per cent had hydrocephalus, but no spasm; 11% had spasm, but no hydrocephalus. Hydrocephalus and vasospasm were significantly associated (P less than 0.01, chi2). These data document a high incidence of mild ventricular enlargement and angiographic vasospasm after subarachnoid hemorrhage. They also emphasize that these two sequelae of subarachnoid hemorrhage are closely linked, probably by the presence of blood in the basal cisterns obstructing cerebrospinal fluid flow and surrounding arteries there.