To elucidate the biological and clinicopathological significance of endocrine differentiation in gastric adenocarcinoma, an immunohistochemical study was made of 127 cases with ascertained five-year survivals, and of 45 recent cases of bromodeoxyuridine (BrdU) labeling. Endocrine differentiated cancer cells were demonstrated in 37 out of the 127 cases (29.1%) evaluated by chromogranin A (CGA) immunoreactivity, and all CGA-positive tumors were classified as advanced gastric cancer. Analysis of retrospective five-year survival rates revealed the adenocarcinomas with endocrine differentiation to have had significantly longer survival times than those without endocrine immunoreactivity in stage II, but not in stages III or IV. Double immunolabeling for CGA and BrdU in the other 45 adenocarcinoma cases showed only a single CGA-positive cancer cell with BrdU incorporation among a total of 454 CGA-positive cells examined. There was no significant difference between the labeling indices of the general cancer population and the cancer cells adjacent to CGA-positive cells. In conclusion, endocrine differentiation of gastric cancer is not uncommon, particularly in advancing cancer, and it would be a useful marker for a better prognosis in stage II. Probably, endocrine differentiated cancer cells are almost dormant with virtually no DNA synthesizing activity, and their paracrine effect is most unlikely to work in vivo.