DNA content was measured by image analysis in a retrospective study of formalin-fixed paraffin-embedded colorectal carcinomas from 213 patients who were followed up for at least 5 yr. DNA histograms were classified as diploid, aneuploid, or tetraploid. Diploid tumors comprised 29% of all cases, aneuploid 50%, and tetraploid 21%. Aneuploid tumors were found more often in patients with advanced disease and in carcinomas arising in the rectum. Pathologic stage, histologic grade, and ploidy were individually related to survival and recurrence. However, after stage stratification, histologic grade was no longer a significant prognostic factor. In patients without regional or distant metastases (Dukes' Stage A and Stage B), patients with aneuploid tumors had a statistically worse prognosis than patients with diploid or tetraploid tumors (P less than 0.01). The prognostic value of ploidy in this group of patients was maintained only in tumors arising in the distal colon and rectum (P less than 0.04). In patients with regional or distant metastases, DNA content did not provide additional prognostic information. In conclusion, DNA quantitation can be evaluated reliably by image analysis of archival material and can provide valuable prognostic information, especially in patients with Dukes' Stage A and Stage B disease. It may prove useful in guiding adjuvant therapy in these patients.