A rapidly rotating single photon tomograph is described for atraumatic and three-dimensional measurements of rCBF in ml/100 g/min. The instrument has a high sensitivity permitting the recording of the uptake and washout of xenon-133 from the human brain during and after inhalation of the gas at a concentration of 10 mCi/l for 1 min. The algorithm used to calculate rCBF is described, and clinical results in particular in stroke cases are presented. The rapidly rotating tomograph is better suited for the study of focal cerebral ischemia than the conventional stationary detectors because superposition of tissue layers is avoided.