Flow cytometric assay of nuclear DNA in endoscopic biopsy specimens was evaluated in colon cancer patients. When the cells were divided into diploid cells and aneuploid cells, aneuploidy was observed in 63% (58 of 92) of the colon cancer patients. However, no clear relation was observed between the frequency of aneuploidy and the invasive depth, size, or histological type of colon cancer. Noncancerous portions of the colon tissues including colon adenoma or normal mucosa were mostly (96%, 87 of 91) diploid. Nuclear DNA content could be analyzed in the fresh biopsy specimens of colon cancer tissues and such investigation might be possibly valuable for further biological characterization of colon cancer in the usual procedure of clinical diagnosis for colonic malignancy before surgical operation or other treatment.