A necropsy study of polypoid lesions and diverticula of the large bowel in the Japanese prefectures of Akita and Miyagi indicated that adenomatous polyps were more numerous in Akita (30%) than Miyagi (18.3%). They also were larger and showed more severe atypia in Akita. The sigmoid colon and rectum showed the most severe atypia in both prefectures. Hyperplastic polyps and diverticula were very uncommon in both prefectures. These results are discussed in relation to the different levels of risk to colorectal carcinoma in these prefectures and in Japanese migrants to Hawaii.