The nuclear DNA content of 163 colorectal carcinomas was determined by flow-cytometry (FCM) on formalin-fixed and paraffin-embedded tissue. DNA-aneuploidy was found in 97 cases (59.5%), without correlation to sex, mean age, tumor-stage (DUKES and pTNM) and grading. The frequency of aneuploidy was statistically significantly higher in patients younger than 70 years of age (p less than 0.01) and in tumors localized in the left colon and rectum (p less than 0.002). The tumors in which different areas could be analyzed (n = 80) showed a heterogeneous DNA-ploidy pattern in 18%. The comparison of DNA-content in primaries and in lymph-node metastases (n = 49) resulted in a difference of DNA-ploidy in 38% of the DNA-aneuploid tumors, but only in 6% of the DNA-diploid carcinomas (p less than 0.02). Carcinomas with DNA-aneuploidy showed a trend to a higher rate of loco-regional recurrences, a higher S-phase fraction (13.5% +/- 5.9 vs. 8.1 +/- 7.0), and proved to be associated with a poorer prognosis (p = 0.04). The statistically significantly higher mortality of patients with DNA-aneuploid carcinomas in DUKES A and B stages indicates that DNA-aneuploidy could perhaps be regarded as a stage-independent additional risk factor.