Out of a series of 43 cases operated on for ruptured intracranial aneurysms over a 12-month-period, 32 patients were followed up to 12 months postoperatively with repeated evaluations of cerebral blood flow (a mean of 3.4 studies for each patient), using the Xenon133 inhalation technique. Percent variation of hemispheric CBF from the sex- and age-matched reference values, obtained in both affected and contralateral side at 60-90 days postoperatively, was calculated in each of the patient assigned to one subgroup [a) patients submitted to delayed surgery; b) patients submitted to early surgery with a WFNS score of I-III, and c) patients submitted to early surgery with a WFNS score of IV-V]. The percent differences were averaged and the resulting mean values compared. None of them showed statistically significant differences, suggesting that the timing of surgery did not affect the results, either in good and poor grade patients, as far as cerebral blood flow is concerned. Depression of flow in the affected hemisphere of poor-grade patients was principally related to the preoperative occurrence of an intracerebral haematoma.