The application of recursive filtering in digital radiology can yield incorrect blood flow estimates if blood flow is calculated using a transit time, defined as the difference in times to peak contrast over the length of a vessel segment. The transit time error is calculated here, from computer simulations, as a function of filter design, image noise and contrast medium dispersion over the vessel segment. An error reduction procedure based on prefiltering is suggested and tested by simulation. Results indicate that the transit time error can be substantial and the reduction procedure effective, depending on the filter design and system noise level.