Results in 44 cases of malignant tumors treated by localized radiofrequency hyperthermia are presented. All patients had either one or more failed attempts at conventional therapy or had such advanced disease at presentation that control with conventional modalities was considered unlikely. Hyperthermia were used together with radiation in 42 patients with chemotherapy in 1 patient, and alone in 1 patient. The overall response (CR plus PR) was 50% including a 18% complete response. Tumor response was somewhat better for superficial tumors. We retrospectively have analyzed the association between tumor response and histology, tumor size, temperature, radiation dose and total number of heat sessions. Clinical factors influencing the tumor response were initial tumor size and intratumor temperature. The incidence of overall response increased with a decrease in initial tumor size and an increase of intratumor temperature.