The preoperative concentrations of IgG were lower (P less than 0.002) and the concentrations of C4 and C1-INH higher (P less than 0.01 and P less than 0.001) in 29 patients with recurrence after potentially curative resection of gastric carcinoma, than in 31 patients alive and disease-free 5 years after surgery. These differences between the two groups of patients were consistent within each of six groups of disease extent. In each of the two groups of patients, the preoperative concentrations of IgG, C4 and C1-INH had no significant variation with the extent of disease (P greater than 0.05 or greater). Of our variables, C1-INH was the most potent prognosticator and discriminated between patients with and without recurrence with 80% accuracy. Furthermore, the predictive prognostic value of C1-INH at the time of surgery was superior to the prognostic value of the extent of disease (F values 27.00 and 12.69). Apparently, the preoperative C1-INH concentration is an essential and independent prognostic parameter of gastric carcinoma. We assume that C1-INH reflects an additional prognostic feature appropriate to the tumour or the host. Our finding that the interval between surgery and death from recurrence had an inverse relation to the preoperative C1-INH concentration also supports this assumption.