Striking differences in outcome of surgically treated colon adenocarcinomas were observed in 33 patients according to the DNA distribution patterns in tumor nuclei, measured by flow cytometry. As discussed and defined in the text and confirmed by appropriate control studies, the tumors were classified into 2 groups: predominantly diploid (20 tumors) and predominantly nondiploid (13 tumors). During the follow-up period of 3-5 years, 12 of the 13 patients with "nondiploid" tumors died of disease within 4-34 months, and the 1 patient still alive after 59 months has extensive metastases. Only 6 of the 20 patients with "diploid" tumors died of disease, sometimes after a slow, protracted clinical course. There were 14 patients with no evidence of disease for periods ranging from 30 to 60 months; 1 of these patients died of other causes. Histologic grading and Dukes' staging appeared to play a relatively limited role in the outcome, although lymph node metastases and vascular invasion were more often observed in the nondiploid tumors. These observations suggest that the determination of DNA distribution in colon carcinomas may prove to be of prognostic value.