Computed tomography (CT) provides explicit information for tumour localisation. However, CT data are displayed in the transverse plane, whereas on the Radiotherapy Simulation films, radiation portals are commonly directed into anteroposterior and lateral planes. Using present day radiotherapy planning computers, it is possible to read in the CT data to determine the tumour volume in anterior/posterior and lateral directions and to perform dosimetry calculations. In cases where the digital computer data cannot be used directly by the radiotherapy planning computer, (for example, owing to compatibility problems) the radiation oncologist must transfer information about tumour location from the CT hard copy images to the longitudinal planes of the simulation film by hand. This manual data transfer can now also be performed using the new personal computer programme, Digiplot, which we have developed and is described below. The application of Digiplot to the head and neck area has shown that it complies with the accuracy requirements laid down by the radiation oncologist. The clinical impact of Digiplot was tested on ten patients with a brain tumour. In two cases, Digiplot detected errors which had not been observed using the conservative (manual) method.