A multivariate probit analysis was used to develop a prognostic scoring system known as the Prognostic Index (PI), which consisted of age, pulse rate, blood urea nitrogen, serum albumin, serum cholesterol and serum potassium. This PI accurately predicted the severity and mortality of 83 surgical patients with septic shock caused by gastrointestinal diseases, who were admitted to Tenri Hospital, Nara, Japan from 1975 to 1981 (internal check), and also that of 25 other patients, examined from 1982 to 1983 (external check). As the prognostic predictive model provides a prompt and easy prediction of how severe the patients' state is, it has proved to be very useful to our surgical staff for managing critically ill patients.