Cellular DNA ploidy of paraffin-embedded tumor tissues from 51 patients with rectal cancer less than 3 cm in diameter was determined by flow cytometry. The relationship of DNA ploidy to Dukes stage, histological grade, lymph node metastasis and to the survival rate of postoperative patients was analyzed. The incidences of diploid tumors in patients with Dukes stage A, B and C tumors were 83%, 52% and 21%, respectively. There were significantly more patients with lymph node metastasis in the nondiploid cancer group than in the diploid cancer group. Patients with diploid cancer had significantly better 5-year and cumulative survival rates than did patients with nondiploid cancer. The results suggest that analysis of cellular DNA ploidy can provide a new prognostic parameter for rectal cancer less than 3 cm in diameter.