The proliferative activity of gastric adenomas from 18 patients (42 endoscopic procedures) was compared with follow-up results. These cases were gastric adenomas proven by follow-up with repeated endoscopic procedures for more than 2 years, or were confirmed as gastric adenocarcinoma thereafter by histopathologic examination. Among the eighteen cases, nine showed carcinoma in the subsequent biopsies (group 1) and the remaining nine did not result in carcinoma (group 2). The proliferating cell nuclear antigen (PCNA) positivity rates of the two groups were significantly different (P < 0.01). The average PCNA positivity in group 1 was 33.1%, while it was 10.0% in group 2. The risk of developing carcinoma increased as the PCNA positivity increased: 0% in the low PCNA positivity group, 41% in the mid-positivity group and 89% in the high positivity group. We concluded that growth fraction could be taken into account as one of the most important prognostic factors for gastric adenoma, and accordingly repeated endoscopic biopsies with close follow-up should be carried out especially in the high PCNA positivity group.