Flat adenomas in a colon cancer-prone kindred.

We describe new pathologic findings in a hereditary nonpolyposis colorectal cancer family. Affected family members developed multiple small adenomas with right-sided predominance; many adenomas had an unusual appearance featuring slightly elevated lesions with adenomatous changes confined to the upper regions of the colonic crypts. We have adopted the previously established term "flat adenoma" for these lesions. This phenotype may be a morphologic marker for at least one subset of hereditary nonpolyposis colorectal cancer.