A total of 307 patients with superficial measurable tumors were registered on a Radiation Therapy Oncology Group (RTOG) protocol involving fractionated radiation therapy, either alone or followed immediately by hyperthermia (42.5 degrees C, 45-60 min). Overall complete response (CR) was observed in 30% of the lesions treated with radiotherapy (RT) and 32% of those receiving RT and heat. Response was found to be significantly related to both maximum tumor diameter (less than 3 or greater than or equal to 3 cm) and site/histology (breast/adenocarcinoma, head and neck/squamous, or other site/histologies). In tumors less than 3 cm in diameter in the breast, trunk, and extremities, a better CR rate was noted with irradiation and heat (62 and 67%) than with irradiation alone (40 and 0%). However, in the head and neck there was only minimal difference in CR with irradiation alone or combined with hyperthermia (50 vs 38%). In lesions less than 3 cm treated with irradiation and heat, there was improved local control. In lesions greater than 3 cm, there was no difference in local control between the two treatment arms. The higher response rate in patients with smaller lesions (less than 3 cm) may be explained by the fact that these tumors are easier to heat. Problems in correlating tumor response with quality of heating include less than optimal heating in larger lesions and the limited ability of current thermometry to map the temperature distribution in a tumor. Acute and late toxicities in both treatment arms were comparable, except for an overall 30% incidence of thermal blisters in the heated tumors.