Clinical outcome of seriously ill surgical patients with intra-abdominal infection depends on both physiologic (APACHE II score) and immunologic (DTH score) alterations. The delayed-type hypersensitivity (DTH) response and the APACHE II score in 118 patients with surgical infections were measured prospectively and related to outcome. Logistic regression analysis generated the equation: [formula: see text]. The risk assessment as calculated by this model was compared to that using the APACHE II system alone in a separate group of 354 patients. There was an improvement in the predictive capacity of the APACHE II + DTH equation compared to APACHE II alone, as shown by a better fit of expected and observed deaths, an improved Goodman-Kruskal G statistic, and a larger area under the receiver operating characteristic curve. It is concluded that the DTH response (a broad marker of immunocompetence) is an independent prognostic factor in surgical patients and can be used in combination with the APACHE II score (a measure of acute physiology) to estimate better the outcome of surgical patients. 