Stage B adenocarcinomas of colon (61 carcinomas) and rectum (44 carcinomas) were retrospectively reviewed in order to define prognostic indicators and to determine if they are different for tumors arising at the two sites. Parameters evaluated included the substage of the tumor, histologic grade, presence of vascular/lymphatic invasion, residual adenoma or extracellular mucin, and tumor cell DNA content as determined by flow cytometry of paraffin-embedded tissues. Five-year survival was 81.0% for patients with colonic and 87.5% for those with rectal tumors. DNA ploidy was a significant predictor of overall and disease-free survival in patients with rectal adenocarcinomas (improved survival for those with diploid tumors) but not in those with colonic tumors and not in the two groups combined. The degree of differentiation of the tumor was prognostically significant in the colonic group and for the groups combined but not in the rectal group. There was a trend toward better survival in substage B1 tumors, as compared with substages B2 and B3, particularly in rectal neoplasms. Other histologic parameters did not predict survival. We conclude that the prognostic indicators in colonic adenocarcinoma seem to differ from those of adenocarcinoma arising in the rectum and that DNA ploidy has a significant impact upon outcome in stage B rectal adenocarcinomas.