The difference in tracer arrival times between the radial artery and the brain following i.v. injection of 15O-labeled water plus the difference in dispersion of the tracer bolus between these two sites have to be accounted for in order to quantify cerebral blood flow by the autoradiographic approach and positron emission tomography (PET). We describe a method that simultaneously corrects for these two effects by means of a four-parameter fit to the dynamically acquired data. Unlike with other methods, where the two corrections are performed sequentially, no additional measurement of the dispersion time constant is required. We have validated and tested the method by means of simulations and application to data from six human studies. The mean dispersion time constant of 4.0 +/- 1.2 sec, estimated by the new method for the six studies, is in fair agreement with estimates of 3 to 5 sec derived from cardiac PET.