Sixty-two patients with 24 primary advanced, six persistent, 28 locally recurrent, and four metastatic tumors of the head and neck were treated with combined interstitial low-dose iridium-192 radiation therapy, interstitial 915-MHz microwave hyperthermia (IHT), and external-beam radiation therapy. Diagnoses were squamous cell carcinoma in 56, adenocarcinoma in three, and soft-tissue sarcoma in three lesions. IHT was applied immediately before Ir-192 was placed and after its removal for 45-60 minutes at 41 degrees C-44 degrees C. At 3 months, complete remission had occurred in 39 lesions; partial remission, in 18; and no change or progressive disease, in five. At 12-month follow-up, local control was achieved in 29 of 50 patients; seven other patients had slow ongoing tumor regression with an unclear residual mass at computed tomography or magnetic resonance imaging. Lesion type, tumor volume, total radiation dose, and thermal parameters with "good quality of heating" at high minimum tumor temperature were identified as statistically significant (P less than .05) prognostic factors influencing initial and long-term tumor response. There was no prognostic factor for acute or late thermal damage.