The thermal enhancement of radiation-induced damage in pig skin has been investigated. Heating at 43°C for 60 min was produced by a scanned 3MHz ultrasound transducer, immediately after single doses of X rays. The ED 50 values for the dermal reactions of dusky/mauve erythema and necrosis after irradiation alone were 18.6 ± 0.5 Gy and 20.5 ± 0.4 Gy, respectively. The reduction in the ED 50 values to 15.3 ± 0.4 Gy and 17.7 ± 0.5 Gy after irradiation plus heating was significant and suggested a thermal enhancement ratio (TER) of between 1.15 and 1.22. These TER values were within the range obtained in both pig and rat skin using other ‘dry’ heating methods. This would suggest that the non-thermal effects of ultrasound do not influence the thermal enhancement of x-irradiation damage.