A prospective study of delayed hypersensitivity was carried out in gastric cancer patients. One hundred and fifty-six subjects were studied. Fifty-nine were controls and ninety-nine patients with gastric neoplasm. A multitest technique was used to evaluate the delayed hypersensitivity response, classifying the subjects into one of three groups: immunocompetent, relatively anergic, and anergic. In the control group, 76% per cent of the subjects were immunocompetent; of the patients, only 43% showed an adequate cell-mediated immune response (p less than 0.001). Fourteen control subjects (24%) and 54 patients (56%) presented with either anergy or relative anergy (p less than 0.001). Of the ninety-four patients that underwent surgery, five (11.9%) of the immunocompetent group developed major postoperative septic complications, as did seven (22%) of the relative anergic and seven (35%) of the anergic patients (immunocompetent vs. anergic, p less than 0.05). Our study indicates a relationship between anergy and postoperative septic complications.