The colon and rectum were removed from 445 consecutive autopsies and examined for the presence of polypoid lesions under an illuminating magnifying lens. There were seven clinically undiagnosed carcinomas, 329 adenomas, 174 hyperplastic polyps, 59 mucosal tags, 34 other polypoid lesions, and 54 polyps in which no histologic diagnosis could be made. Adenomas were present in 34% of the men and 32% of the women. The adenoma prevalence rate increased smoothly with age in men, whereas the rate in women showed a biphasic variation with a peak at 50--59 years followed by a second increase after 70 years of age. The distribution of adenomas within the bowel showed a marked change with age, from a moderate left-sided predominance before 60 years of age to a marked right-sided predominance in patients over 80 years of age, particularly in men. Hyperplastic polyps were present in 27% of the men and 18% of the women. In men, their frequency and distribution within the bowel showed similar changes with age as were seen for the adenomas. In women, no such age-dependency was observed for hyperplastic polyps. Multiplicity of polyps increased with age and was more pronounced in men than in women. Adenomas in women tended to be larger, more atypical, and more villous than in men. A comparison with a similar study from Northern Norway showed no difference in the prevalence of adenomas in spite of a 70% higher incidence rate for colorectal cancer in Oslo.