Temperature profiles and depth data have been measured in three different tissue-type phantoms to assess the induced thermal patterns from a modified diathermy applicator with no allowance made for blood flow. Results indicate that tumours up to 50 mm in diameter and 30 mm deep may be therapeutically heated. Increasing the fat thickness has been shown, by these phantom studies, to indicate the need for careful regulation of the surface temperature to reduce large thermal gradients at depth. Field mapping outside the treatment area showed no significant exposure to patient or operator. The applicator is presently being used for a clinical hyperthermia treatment study.