A clinical hyperthermia system using a 915 MHz microwave generator and incorporating multipoint thermocouple thermometry is described. Temperatures can be monitored simultaneously at 16 points and measurements displayed on a visual display unit and a plotter. The power output of the generator is adjusted under computer control to maintain a constant predetermined temperature in a chosen control channel. A clinically useful feature of the system is the ability to determine the effective cumulative thermal dose delivered to the tissue at points monitored in real time. The basis for the thermal dose calculation is discussed in detail. The calculated dose parameter is displayed for each of the 16 channels during the treatment and updated every 30 s. This real-time display of a thermal dose parameter has made possible a more homogeneous heating of tumours.