A medical diagnostic decision support system (DDSS) has been developed for and tested in general practice. Two major issues have been addressed: diagnostic support and usefulness. The diagnostic support pertains to the ability of the system to generate diagnostic hypotheses from a set of patient data. The usefulness is approached by creating a computer system which can be used simultaneously with the doctor-patient consultation. The support function operates by matching symptoms from the patient data base with symptom configurations contained in the knowledge base. The support is presented as a list of diagnostic hypotheses ranked by degree of concordance. A user-friendly interface has been constructed with a comprehensive set of clinical terms within which the doctor can locate a desired symptom and store it with a single keystroke. With another keystroke the doctor can check the stored data and ask for support at any moment during the process. The overall purpose is to invite the doctor to rethink and re-examine his steps and to reconsider possible alternatives in the light of the presented diagnostic information. In our view it has to be the doctor who makes the final judgement. A test with the system in general practice revealed good performance of the system and an astonishing proficiency of the participating doctors in its use during the consultation. Twenty doctors solved five patient cases, entering 2000 clinical items within acceptable limits of consultation time. In 96% of the cases the correct diagnosis appeared in the differential diagnosis list. The doctors' diagnostic accuracy was 43%. The use of standardised terminology as an option for further development is discussed. The role of the doctor in computer-aided diagnostics remains open to debate. A computer-aided diagnostic support system in general practice appears to be feasible.