Obtaining benefit from the use of hyperthermia in treating cancer is strongly dependent on the success of the heat treatment in raising the temperature adequately throughout a tumour volume (Dewhirst & Sim, 1984). Interstitial hyperthermia offers one of the best methods of uniform heat delivery (Doss & McCabe, 1976). The radiofrequency (RF) system described here is relatively simple and integrates well with interstitial brachytherapy treatment using afterloaded iridium-192 wires. Under general anaesthesia, hollow stainless steel needles (1.8 mm o.d.) are implanted into the tumour using a central trochar. The desired arrangement of needles is two parallel planes of equal length, evenly spaced, with 1 cm between planes and between needles. Perspex strips or plastic tubing with pre-cut holes are used as jigs to ensure appropriate geometric arrangement, since unacceptable overheating will occur wherever two adjacent needles converge.