For the hyperthermia treatment of human cancers considerable advantage would be gained from the ability to map temperatures deep within the body by non-invasive methods. This could be achieved by the measurement of some parameter that is sensitive to temperature change but insensitive to all other influences. Such a system must also be capable of operating in strong EM fields and be easily accommodated within all hyperthermia treatment configurations. A new thermal imaging technique by mapping electrical conductivity changes with temperature has been investigated. Temperature changes could, in principle, be deduced from this measured data if the temperature coefficient of conductivity for tissue (typically 2% per degree C) is known. Experimental studies have been performed with a 16-electrode system using an agar phantom (conductivity 2.0 S m-1). Heating of a small area of the surface is achieved using a 2 cm diameter microwave diathermy applicator. A time series of images readily shows the distribution of thermal changes and suggests a temperature resolution better than 1 degree C. In vivo experiments were performed by heating the scapula region of a volunteer while monitoring the induced temperature change with the same electrode configuration affixed to the skin surface. These results show a greater degree of image artefacts compared with the phantom results, possibly caused by a combination of body movement, blood flow effects and electrode contact degradation. These initial experiments have demonstrated the feasibility of non-invasive thermal mapping using electrical impedance imaging. Phantom studies have shown the possibility of displaying the thermal distribution induced by microwave applicators for both profile and depth planes in tissue.(ABSTRACT TRUNCATED AT 250 WORDS)