The usefulness of a heat sensitivity test which involved performing an ATP assay on endoscopic biopsy materials for predicting the clinical response to hyperthermia was investigated in esophageal cancer patients. Following in vitro heat treatment of FM3A tumor cells, the heat sensitivity detected by ATPA was significantly correlated with that in the colony-forming assay, and the percent inhibition of the ATP level in the tumor cells was correlated with in vivo tumor growth. The heat sensitivity of the biopsy materials evaluated by ATPA correlated well with that of the resected specimens in 18 esophageal cancer patients, while the clinical response to thermal therapy was well predicted by the heat sensitivity of the biopsy materials evaluated by ATPA in seven of the patients. These results indicate that the heat sensitivity test conducted by performing an ATPA on endoscopic biopsy materials could be a useful indicator for predicting the clinical response to thermal therapy.