Thirty-seven patients with 18 primary advanced or persistent, 16 local recurrent, and three local metastatic tumors of the head and neck were treated with a combination of interstitial low-dose iridium-192 radiation therapy and interstitial 915-MHz microwave hyperthermia supplemented by external radiation therapy. Twenty-eight lesions received an additional external radiation dose of 21-61 Gy. Interstitial hyperthermia was applied immediately before Ir-192 was placed and after its removal for 45-60 minutes at 41 degrees C-44 degrees C. Follow-up ranged from 4 to 45 months. At 3 months, complete remission occurred in 25 lesions (68%); partial remission, in nine (24%); and no change or progressive disease, in three (8%). At 12 months of follow-up in 32 lesions, local control was achieved in 23 (72%), with the patients alive, and in four (12%), with the patients dead. There were five local recurrences, one of which occurred after complete response. Lesion type, tumor volume, radiation dose, and thermal quality at high minimum temperature were identified as prognostic factors influencing complete remission. The combined treatment was well tolerated.