An automated procedure to help general practitioners in clinical diagnosis and decision making is presented. The computer-based program is conceived to process results from laboratory tests performed on outpatients, providing general practitioners with possible causes of abnormal results. When only one or two abnormal tests are observed, a series of suggestions pertinent to each abnormality is printed. When there are more abnormal test results, the program performs a more complex procedure ending with the output of some diagnostic hypotheses. Messages are also printed to focus the physician's attention to particular aspects of patient pathology that were sometimes missed or disregarded and to suggest new investigations the laboratory can perform to improve diagnostic efficiency. Moreover some advice is supplied to allow a better evaluation of particular risk conditions, as those associated with the development of coronary heart disease. The program has been recently extended with the calculation of intraindividual reference intervals. The system described has been working since 1976 and appears particularly useful when the general practitioner is faced with a number of pathological results of difficult interpretation.