Forty-seven colorectal 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, 6 of 47 polyps (12.8%) contained DNA aneuploid cells, comprising 4 adenomatous polyps, 1 of 12 with mild atypia (8.3 %), 2 of 19 with moderate atypia (10.5 %) and 1 of 4 with severe atypia (25.0%), and 2 of 4 polyps (50.0%) with histologic features of carcinoma in situ (CIS). The incidence of DNA aneuploid cells in CIS was significantly higher than that of all adenomatous polyps (p = 0.043). All of the 8 hyperplastic polyps were diploid. The detection of aneuploidy as a marker of increased risk for malignant transformation was possible even in colorectal adenomas measuring ≤ 10 mm. These results show that retrospective FCM analysis can detect DNA aneuploidy in adenomatous polyps and that DNA aneuploidy may occur before histologic evidence of invasive carcinom