Flow cytometry was performed upon 312 patients with adenocarcinoma of the colon and rectum, satisfactory results being obtained with 275 (108 diploid, 130 aneuploid and 37 tetraploid). The proportion of nondiploid instances increased from 28 percent if one, to 80 percent when six specimens were assessed per patient. Reproducibility of the technique showed substantial agreement in the assessment of deoxyribonucleic acid ploidy (Kappa value equals 0.74). Increasing values of cells in the diving (G2/M) phase of the cell cycle were associated with little lymphocytic tumor infiltration (p = 0.0002) and extensive tumor fibrosis (p = 0.003). Univariate survival analysis revealed that, although diploid tumors tended to have a better prognosis than nondiploid tumors (p = 0.06), no flow cytometric variable was significantly related to survival. Flow cytometry similarly was not of prognostic value in instances without lymph node metastases or without distant metastases. Multivariate regression analysis of flow cytometric and clinicopathologic variables identified Dukes' stage, patient age and tumor differentiation as the combination of variables most closely related to survival. No flow cytometric variable could significantly improve on the prognostic model containing these three variables. It is concluded that conventional histologic variables remain the best predictors of prognosis in carcinoma of the colon and rectum.