The aim of this study was to re-evaluate the clinicopathologic features of flat adenomas with special reference to the role of central depression found in flat adenomas. Clinicopathologic features, such as grade of atypia by size, site, central depression, coexisting lesions, gender, and family history of cancer, were evaluated in 236 flat adenomas from 183 patients selected retrospectively and prospectively. Of the 236 flat adenomas, 175 had mild, 33 had moderate, and 28 had severe (i.e., intramucosal carcinoma) atypia. The frequency of severe atypia correlated positively with size. Severe atypia were significantly more frequent in females (21 percent in females and 10 percent in males; P < 0.05). Sixteen percent of flat adenomas in patients with coexisting cancer showed severe atypia, which was significantly more than those without coexisting cancer (P < 0.05). The frequency of multiple flat adenomas in patients with two or more family members with cancer was 53 percent, which was significantly higher than in patients with one or no family members with cancer (P < 0.05). Central depression was observed in 19 percent of flat adenomas. The rate of severe atypia (22 percent) of flat adenomas with central depression was significantly higher than that (9 percent) of flat adenoma without central depression (P < 0.05). Patients who had flat adenomas with central depression had a higher incidence of coexisting cancer and cancer in the family (P < 0.05 and P < 0.01). At colonoscopic examination consideration should be given for the increased potential to become malignant in flat adenomas, especially those with central depression and in patients with a history of malignancy or with a family history of malignancy.