The mucosal surface structure of epithelial neoplasms in the large intestine was examined by a stereomicroscope, and the correlation between histologic and macroscopic features was studied. The surface structure of the colonic mucosa could be readily and clearly observed using the stereomicroscope after superficial staining, and neoplastic lesions were identified as well-defined foci of the unique structures that were conspicuously different from those in the normal mucosa. Five hundred two adenomas and 159 carcinomas were examined and divided into five types according to stereomicroscopic appearance: long ellipsoid (359 lesions), cerebriform (141 lesions), leaf-like (16 lesions), dense tiny pits (89 lesions), and devastated (56 lesions). In the first three types benign adenomas predominated in the majority of cases (94.7%, 72.3%, and 68.8%, respectively) and the variation in the surface features correlated well with the histologic types. In the latter two types the frequency of malignancy was strikingly high (49.4% and 92.9%, respectively), with the devastated mucosal appearance being highly indicative of invasive carcinoma. Macroscopically, exophytic polypoid configurations were frequently found in the first three types, whereas flat nonpolypoid configurations predominated in the cases with dense tiny pits. Our results indicate that there is a close relationship between the surface structure of epithelial neoplasms in the large intestine and their histologic features and growth patterns.