The cellular DNA content was measured with flow cytometry from paraffin-embedded materials in 197 patients with primary rectal cancer who had had "curative" resection. All patients were followed clinically for from 2.5 to 7 years. There was a significant difference in recurrent rate between DNA diploid and aneuploid tumors. The cumulative survival rate (Kaplan-Meier) of curative operative rectal carcinomas was worse in DNA aneuploid than in DNA diploid tumors (p less than 0.05). These data suggest that tumor DNA content is an important prognostic indicator for both recurrence and overall survival in patients with primary rectal carcinoma.