Where the attenuation in the basal cisterns was less than that of the surrounding brain, the scan was classified as low density. Where the basajl cisterns had an attenuation similar to that of brain, the scan was classified as isodense, and where the attenuation exceeded that of brain, the scan was classified as high density. The timing of the three groups oif scans in relation to the bleed is shown in fig 1, the majority of patients being scanned within a week of SAH. Fou.rteen patients bled twice prior to surgery, and three bleid on three occasions. In these patients the scan was taken following the last bleed prior to surgery.All patients were given a clinical grading pre-operatively on the five point scale of Hunt and Hess,3 and the average grade of the patients in each scan group was computed. Evidence of pre-existing hypertension wa.s sought in each patient, by a study of their previous medical history, drug therapy prior to the ictus, and the appear.ance of the vessels in their fundi. This information was available i.n 68 patients, and the hypertensive history was correlated with the scan