To identify characteristics of gastric cancer associated with hereditary non-polyposis colorectal cancer (HNPCC), we gathered clinical data and tumor samples relating to patients recorded in the Finnish HNPCC registry. Our series included 51 families with a characterized mutation and/or that met the Amsterdam criteria. Of 570 members affected by malignancy, gastric cancer occurred in 62. Adequate clinical data were obtained for 45 patients. Tumor samples from 24 patients were re-examined. The mean age of diagnosis of gastric cancer was 56 years. The average percentage of all cancers within a family was 11 (range 0-40). Nineteen were of the intestinal type. Only 3 were of the diffuse type. Helicobacter pylori infection was demonstrated in 3 of 15 cases. Replication error (RER) phenotype was present clearly in 7 cases and at least fairly clearly in 11. The overall 5-year survival rate was 15%. The 5-year survival rate was 48% in cases in whom radical surgery had been undertaken. Our results support the view that gastric cancer belongs to the tumor spectrum of HNPCC. The intestinal type of histology is characteristic, as is the RER+ phenotype, but H. pylori infection was rare.