In order to provide a common basis for treatment decisions and the future evaluation of breast cancer therapy, the breast cancer protocol used in our clinic was formulated for the computer. A system for the simulation of clinical reasoning was used which allows the physician to express the medical logic as a series of inferences to be confirmed or rejected on the basis of Boolean combinations of medical criteria. The system enables the physician to rearrange these criteria into a questionnaire for complete and accurate recording of patient data. These data are then fed into the computer to produce a patient status report and treatment recommendations. The data from 26 patients in various stages of breast cancer have been processed by the computer and the computer's conclusions compared to the assessment made by a junior physician in our department. In three cases the staging was not identical, in 15 cases the treatment decisions were not identical and in all cases investigations were left out by the physician. The system has been of considerable value in standardising patient examination and treatment and in training inexperienced physicians.