The authors studied the familial occurrence of tumors in 154 individuals with gastric cancer by reviewing the clinical data and the genealogical tree of all patients registered in 1986 through 1987 in the Local Health Care District of Modena, Italy, for cancer of the stomach. Crude and age-adjusted (world population) incidence rates of gastric cancer were 34.0 and 21.4 new cases/100,000/year, respectively, in men, and 24.5 and 10.9 in women, respectively. Among first-degree relatives of the registered patients there were 30 cases of gastric carcinoma versus 15 cases in a control group matched for age and sex (Mantel-Haenszel odds ratio [M-H OR] 3.14, P less than 0.01). This excess of gastric neoplasms was observed in siblings (17 versus 7, M-H OR 4.33, P less than 0.02) but not in parents (13 versus 8, not significant). Besides gastric cancer, there was no significant excess of other type of tumors in case families. The familial occurrence of gastric cancer tended to be more frequent in patients with "diffuse" carcinoma (52%) than in subjects with "intestinal" cancer (33%), although the difference was not statistically significant. In conclusion, the current investigation suggests that a "family history" for gastric neoplasms is usually observed in approximately 10% to 15% of the registered cases. As already described for other common malignancies, therefore, the familial occurrence of gastric carcinoma suggests the existence of a genetic susceptibility to cancer of the stomach, at least in a fraction of these patients.