Twenty locally advanced lung cancers were treated by hyperthermia in combination with radiotherapy between November 1980 and January 1990. All tumors selected had invaded or were in contact with the chest wall, so that transcutaneous insertion of thermal probes into the tumor was possible. Using an 8 or 13.56 MHZ RF capacitive heating device, hyperthermia was given once or twice a week after irradiation for 30-60 min per session (1-12 sessions in total). Radiotherapy was delivered at dose of 13.6-70 Gy. The thermal parameters analyzed were a) maximum, average, and minimum intratumor temperatures (Tmax, Tav, and Tmin), which were recorded at the termination of each treatment, and b) the percentages of the intratumor points that exceeded 41 C (%T greater than or equal to 41 C). The mean +/- SD for Tmax, Tav, Tmin, and %T greater than or equal to 41 C was 42.9 +/- 1.7 C, 41.6 +/- 1.2 C, 39.7 +/- 1.1 C, and 56.2 +/- 25.8, respectively. Larger tumors showed higher thermal parameters than the smaller tumors. Of the 12 tumors treated by definitive therapy, 2 (17%) achieved CR, 7 (58%) PR, and 3 (25%) NR. Four of 10 tumors that did not achieve CR showed large intratumor low density areas on post-treatment CT, reflecting massive coagulation necrosis. Higher thermal parameters were closely related to the appearance of low-density areas but not to changes in tumor size. Four tumors treated preoperatively were successfully resected 2 weeks after thermoradiotherapy, whereas four palliatively-treated tumors showed no regression. The side effects associated with hyperthermia were pain in 12 patients (60%) and dyspnea in 3 (15%), all of which resolved after termination of treatment. A skin abscess and a pneumothorax attributed to thermal probe insertion were observed in one patient each. These results indicate that regional RF capacitive hyperthermia is clinically feasible for local treatment of selected lung cancers.