Correlation between hyperthermoradiosensitivity and clinical effect in carcinoma of the esophagus. The correlation between hyperthermoradiosensitivity evaluated by an in vitro succinate dehydrogenase inhibition (SDI) test and the histopathologic effects of hyperthermochemoradiotherapy (HCR therapy) were investigated in 43 patients with carcinoma of the esophagus. The succinate dehydrogenase (SD) activity of tissue fragments taken at biopsy was assayed after exposure to heat (43 degrees C.) and radiation (6 grays) was done. The sensitivity to radiation plus heat treatment was estimated by the percentage of SD activity of the treated cells, compared with that of the control cells. The 43 patients were divided into three groups according to the degree of SD activity after exposure to radiation plus heat treatment. The SD activity was less than 50 per cent in group 1 (highly sensitive), between 50 and 70 per cent in group 2 (moderately sensitive), more than 70 per cent in group 3 (less sensitive). Eighteen of 20 in group 1, 11 of 17 in group 2 and two of six in group 3 were classified as being histopathologically "effective" for HCR therapy. The two year survival rate for groups 1, 2 and 3 were 55.5, 34.9 and zero per cent, respectively, while there were no statistical differences with regard to prognostic factors. These data suggest that in vitro activities of SD correlate well with the clinical effectiveness of HCR therapy. Therefore, it is recommended that a SD inhibition test be included among the guidelines for clinical management. 