We used in vitro succinate dehydrogenase inhibition test (SDI test) to predict the combined effects of hyperthermia and irradiation on specimens of esophageal cancer obtained at endoscopy. The mean activity of succinate dehydrogenase (SD) after combined treatment of 6 Gy of irradiation and 43.0 degrees C for 20 hours of hyperthermia was significantly decreased compared to findings with each single treatment (to irradiation; p < 0.01, to hyperthermia; p < 0.05). A correlation between the SD activity and the clinical remedial value was also examined in tissues from 47 patients with esophageal cancer, and treated with hyperthermia and irradiation, in combination. The clinical effect, determined radiographically, showed a correlation of 76.6%. Thus, hyperthermia plus irradiation is effective clinical treatment and the SDI test facilitates prediction of the outcome of this combined treatment.