A case-control study in the population of the Eastern Black Sea region of Turkey was conducted to learn the incidence of stomach cancer in the siblings of patients with gastric carcinoma, Among 1240 patients with gastric carcinoma, 168 had sibling(s) with a history of stomach cancer versus 19 cases in the control group matched according to age and gender (OR 10.07, P < 0.0001). The frequency of a history of stomach cancers and cancer of other organs in first- to third-degree relatives was 60.7% and 38.0%, respectively, of 168 sibling cases with gastric carcinoma (P < 0.0001). Fifty-two point three per cent of sibling cases having a history of cancer in other organs in their relatives also reported stomach cancer in the same-degree relatives. The number of stomach cancers in the first- to third-degree relatives of sibling cases was higher than the number of other organ cancers in the same-degree relatives (P < 0.01). Familial clustering of stomach cancer was reported in 12.5% of sibling cases. The study of stomach cancer history in the siblings suggests: the presence of a genetic susceptibility, high risk of the disease occurrence in the siblings of patients, higher predisposition to gastric than to other organ cancers in the relatives, and not infrequent familial clustering, (C) 2000 Lippincott Williams & Wilkins.