In order to measure the absolute ventricular volumes in ml we developed a method using the numerical prints of CT (EMI, CT 1000) on which — by comparing with corresponding Polaroid photos — the margin between brain tissue and ventricles was drawn. For further evaluation, a curve digitizer and a digital computer were used. In 51 patients without any abnormal findings in CT, we studied the volume of single ventricles. The mean value of the whole ventricular system was 30.9±5.7 ml. Reproducibility by this method is within a range of 5%.