Tumour and peritumoral mucosa were analyzed using flow cytometry in 21 consecutive patients operated on for colorectal carcinoma. Thirteen tumours were DNA aneuploid. No foci of aneuploidy bordering the tumours were detected, regardless of the tumour type. On the other hand, a significant decrease in the proliferative index was observed adjacent to the DNA aneuploid tumours. This study provides an additional argument for segmentary excision in colorectal cancers. Complementary studies are still required to specify the mechanisms of proliferation inhibition in the area of the DNA aneuploid tumours.