Eleven patients presented with a second primary cancer during follow-up after surgery for gastric carcinoma. In these patients the serum concentrations of C1-INH and IgG prior to gastric cancer surgery were similar to those of 53 patients with recurrence of gastric cancer. In these two groups, the preoperative C1-INH concentrations were higher and IgG lower (P less than 0.001 and P less than 0.05) when compared to 36 patients alive and disease-free 5 years after surgery. The median time between surgery and signs of recurrence was 11 months, whereas the median time until signs of the second primary cancer was 4 years. A patient with gastric carcinoma who pre-operatively has high C1-INH and low IgG is liable either to have recurrence or to develop a second primary cancer. Our data indicate that these variables represent a cancer susceptibility feature appropriate to the host.