In hereditary non-polyposis colorectal cancer (HNPCC) patients, the cancer frequently arises in the proximal colon and is often multiple (synchronous or metachronous). Pathologic differences seem to exist between hereditary and sporadic large bowel cancer, but the data are not uniform. Many authors reported that the following histologic features are often present in HNPCC: 1) mucinous histotype, 2) poorly differentiated tumors, 3) presence of peritumoral lymphocytic infiltrate, with Crohn's-like lymphoid reaction. Such features have also been found in apparently sporadic colorectal cancer with, but not in sporadic colorectal cancer without DNA replication errors. Many studies have suggested that adenoma plays a main role in HNPCC carcinogenesis, and that the "adenoma-carcinoma sequence" may be the pathway to cancer in HNPCC as in sporadic colorectal cancer. Moreover, HNPCC adenomas show an early onset, villous component, high-grade dysplasia, and positivity for DNA replication errors more frequently than sporadic adenomas. Such data suggest that the adenoma-carcinoma sequence is accelerated in- HNPCC and that surveillance in these patients should therefore be strict to avoid cancer development.