Forty five patients suffering by S.A.H., with I-III degree according to Hunt-Hess classification, have been studied with SPECT, performed within 72 hours from the hemorrhage, in order to evaluate early alterations of cerebral blood flow and their prognostic value. The patients with a clinical worsening showed an important diminution of C.B.F. detected by SPECT and a great amount of blood in the sub-arachnoidal spaces at the CT scan (III degree, according to Fischer). If these results will be confirmed with further studies, it could be possible to early identify a group of patients with a high risk of developing a cerebral vasospasm.