A system of computer-assisted diagnosis is described which, in analogy to conventional diagnosis, takes into account both logical and statistical factors. The arrangement of such a system makes it clear that, after a period of successful use in specialist branches of medicine, a phase of less spectacular success has occurred which, however, provides the conditions for including larger areas of medicine in computer-assisted diagnosis. These systems are likely to conform more closely to the needs of the doctor than previous ones. Whether they can be used in non-hospital practice would depend largely on whether the questions put to the system by the doctor do not take more time and answers can be given in short, easily understandable form. The best type of answer as well as the transmission of information can be discovered empirically. The differential diagnostic level of computer-assisted diagnosis is, according to present experience, quite satisfactory but can be further improved. This development is greatly helped by continuing increases in the speed of calculators and their data storage capacity.