Immunoglobulins (Ig) and some complement components (C) were quantified in sera from patients with gastric carcinoma before surgery and at regular intervals during a 5-year follow-up. The preoperative concentrations of C1-INH and C4 were higher (P < 0.0005 and P < 0.005) and IgG lower (P < 0.0005) in 50 patients with recurrence than in 46 5-year survivors. The prognostic significant of C1-INH was superior to that of the extent of disease (F-values 37.1 and 26.1). The preoperative immune data classified 76% of the patients correctly as to recurrence and no recurrence. Also, the preoperative C1-INH concentration had a highly significant effect on time to recurrence of cancer (P = 0.0007), adjusting for age and disease extent. After surgery the mean IgG concentrations were within normal range and without difference between the two groups. On the other hand, the concentrations of C1-INH and C4 in the individual patients in both groups remained the same from before to after surgery and throughout the observation period (P = 0.34). Apparently, the serum levels of C1-INH and C4 do not reflect the bearing of cancer. We therefore suggest that these variables represent an independent immune state that is appropriate to the host. A comparison of our variables with those of healthy individuals seems to support this idea. This immune state has a significant influence on whether a resected gastric cancer will recur, and also on how soon recurrence may be manifest.