Since inert gas washout techniques for cerebral blood flow (CBF) measurement are not applicable to asymmetrical pathological states where assumptions about constancy of distribution, volume, and partition coefficients cannot be held to be valid, a new approach to regional and global CBF is described. Regional volume and regional mean transit time are measured independently to give regional volume flow in ml/min. In a pilot study, this noninvasive, quantitative technique has been applied to 37 patients with various clinical responses to subarachnoid hemorrhage. Global CBF and clinical grading were significantly associated. Reduction of regional CBF and spasm of the relevant arterial supply were also significantly associated in these patients.