One hundred twenty-eight small flat adenomas (SFAs) were collected from 101 patients, and the clinicopathologic features were investigated. There were 91 adenomas with mild atypia, 20 with moderate atypia, and 17 with severe atypia. SFAs were found more often in males than in females, with a ratio of 3.4:1, and the malignancy rate in females (31.8 percent) was higher than in males (9.3 percent). About 38 percent of the patients had a history of colorectal carcinoma, and 65 percent had a history of colorectal neoplasms. Of 37 patients whose family history was traced, 21 had cancer families. SFAs were prone to be found in patients with a history of colorectal neoplasms and a cancer family. Malignancy rate increased with increasing size. The overall malignancy rate was 13.3 percent, which was considerably higher than that of ordinary small polypoid adenomas (2.8 percent). SFAs were situated more proximally (30.9 percent) than ordinary adenomas; however, there was no relationship between site and malignancy. All the lesions showed tubular adenomas, and there was no villous feature. A central depression was noted in 20 lesions, more frequently in adenomas with higher atypia. All but one adenoma with severe atypia showed a component of lower atypia, supporting the adenoma-carcinoma sequence.