By the clinico-epidemiological approach considering sex and age, ABO blood group and cancer family history data have been examined for 1779 patients with stomach cancer. The results obtained are as follows:<br>In the group of stomach cancer patients with two or more cancer families among their parents and sibs, the percentage of the patients of blood group A is significantly higher than the percentage in the group of control people.<br>When sex and age differences about the former are considered, the rate of blood group A is more prevalent in young male than in young female patients under 50 years old, but more prevalent in old female than in old male patients 50 and over.<br>In the group of young male stomach cancer patients under 50 with one stomach cancer family, the percentage of the patients of blood group A is significantly higher than the finding of the control.<br>These findings also are understood from the analyses of sex ratio considering the age group differences.<br>In the group of young stomach cancer patients with two or more cancer families even in no relation to blood groups, the sex ratio is significantly high in comparison with the sex ratio of all the young patients' group.<br>Concerning to the clinical pictures such as original sites in the stomach and histological patterns, there are no noticeable differences between the stomach cancer patients of blood group A with two or more cancer families and all the stomach cancer patients. But when the age difference is considered, the rate of pyloric antrum cancer is more prevalent in the old patients of the former, and by the morphological examination, it is found that there are some multiple polyposis cancers on the pyloric antrum.<br>Then seeking out the multiple polyposis cancers on the pyloric gland area about 512 cases of early stomach cancers and relatively early stomach cancers whose cancer infiltrations are confined within the submucosa layer, we have found 12 cases of multiple polyposis cancers. Nine cases out of 12 cases are old patients of blood group A, of which, 7 cases are patients with cancer family history.<br>When comparing the original organs of cancer regarding the family cancer patients between the group of stomach cancer patients with one cancer family and the group of stomach cancer patients with two or more cancer families, the rate of the organ cancers, i.e. stomach, uterus, liver and biliary passages, is significantly higher in the latter than in the former.<br>In both groups of the double cancer patients together with stomach cancer and other organ cancer, and the multiple stomach cancer patients (though being restricted to the early stomach cancer cases), both percentages of the patients of blood group A are relatively high as compared to the percentage of the control.