A "host defense index" (HDI) has been developed that predicts the risk of operative mortality based on the impairment of host defense in patients with esophageal cancer. The function of host defense was assessed on admission in 32 patients. Assessment included 21 parameters of nutritional status, cellular immunity, humoral immunity, neutrophil levels, complement levels and serum factors. Operative deaths, which were defined as deaths due to operative complications occurring within 120 days after esophagectomy or by-pass operation, were in 10 out of 32 patients. A discriminant analysis was then performed using 21 parameters in 32 patients, and an equation to calculate HDI was generated. The equation consisted of 10 parameters. The values of HDI more than -0.90 predicted no operative deaths while those less than -0.90 did predict operative deaths. The percentage of patients correctly classified was 87.5%. For clinical application, the predictive risk of operative mortality based on HDI was classified as high (HDI less than or equal to -1.80), intermediate (-1.80 less than HDI less than 0) or low (HDI greater than or equal to 0). The values of HDI in 32 patients were compared with those of the prognostic nutritional index or nutritional assessment index but no significant correlation between HDI and those indices was recognized.