A prospective endoscopic screening examination of a randomized population sample of 200 men and 200 women, aged 50-59 years, enabled the differentiation between individuals with and without polyps in the rectum and sigmoid colon. Both number of years of smoking and percentage years of life with smoking were associated with the presence of adenomas and hyperplastic polyps, both for men and women. No relationship was found between these factors or the daily number of cigarettes and the degree of dysplasia or the size or multiplicity of polyps. Two years' follow-up examination showed a tendency to fewer new polyps among male ex-smokers compared with smokers; however, the difference was not quite significant. No significant difference was found when comparing smokers with never smokers. In the relatively smaller groups of women no significant difference with regard to number of polyps was found among smokers, ex-smokers, and never smokers. The results provide evidence for an association between smoking and colorectal neoplasia and may suggest an initiating effect of smoking on carcinogenesis in the colon and rectum.