Sixty-eight colonic polyps of various histologic types were studied using retrospective flow cytometric (FCM) DNA analysis to determine the prevalence of aneuploid cell populations and whether they were associated with any particular histologic features. Overall, 13 of 68 polyps (19%) contained DNA-aneuploid cells, including 3 of 9 villous and 4 of 10 villoglandular polyps with histologic features of carcinoma in situ (CIS), 4 of 11 villous and 1 of 18 villoglandular polyps without CIS, and 1 of 12 adenomatous polyps. Eight hyperplastic polyps were diploid. These results show retrospective FCM analysis can detect DNA aneuploidy in polyps; DNA aneuploidy may occur before histologic evidence of invasive carcinoma; and this change is more frequent in types of polyps thought to have increased malignant potential (i.e., those with villous morphology and/or CIS).