Ventricular dilatation following spontaneous subarachnoid hemorrhage (SAH) is a well recognized phenomenon. Its clinical significance, however, remains controversial. Two phases are distinguished, the acute or early, occurring soon after the ictus, and the chronic or late, developing after the second week. The authors studied the ventricular size in 210 patients with spontaneous SAH through the course of their illness and convalescence by means of serial computerized tomography (CT) scans. Their findings suggest that ventricular dilatation soon after SAH is not always clinically significant and does not necessarily require shunting before definitive surgery. Delayed symptomatic ventricular enlargement (communicating hydrocephalus) occurs in 7% of the patients and can be safely diagnosed on the basis of the clinical picture and CT scan appearances. Treatment with a ventricular shunting system is almost invariably rewarding.