Flow cytometry was performed on available archival material from 232 patients with rectal cancer enrolled in the National Surgical Adjuvant Breast and Bowel Project protocol R-01. Tumor ploidy was not found to be significantly related to such pathologic and clinical parameters as Dukes' stage; nodal status; nuclear or histologic grades; patient's age, sex, or overall survival rate with an average study time of 79 months. A trend was evident that patients with poor histologic grade, or those with Dukes' B and C tumors that were aneuploid fared worse than those with diploid cancers. However, measurements of survival were found to be more strongly and consistently related to such conventional prognostic parameters as tumor differentiation, Dukes' stage, and nodal status. Further, numbers of nodes with metastases (ie, 1 to 4 or 5+) more significantly discriminated Dukes' C cases than estimation of tumor ploidy. Although tumor ploidy may reflect some features of rectal cancers, their natural history and prognosis are explained better by assessment of conventional parameters used for these purposes.