We studied cell kinetics of colo-rectal neoplasms using PCNA (the auxiliary protein of DNA polymerase-delta) immunohistochemistry. We analyzed the distribution pattern of PCNA positive cells on normal colon mucosa, adenoma (6 cases) and cancer (early: 33 cases, advanced: 6 cases). PCNA positive index (PI) was calculated as the percentage of PCNA positive tumor cells in relation to the total number (about 1000) of the tumor cells. PCNA positive indices were 30% in normal colon mucosa, 49% in adenoma and 72% in cancer, respectively. There were statistically significant differences between three mean values (p < 0.01). In normal colon mucosa PCNA positive cells are localized at the lower part of mucosa, but, in adenoma, they are found at the more upper part of mucosa too. In cancer, PCNA positive cells were localized diffusely and their immunoreactivity is higher than those of the normal mucosa and adenoma. In conclusion, our results suggests that the ratio of PCNA positive cells almost equivalents to growth fraction, and is correlated with the histological grading of colo-rectal tumor, but not correlated with depth of carcinoma. It is presumed that carcinoma has a higher ratio of PCNA positive cells than adenoma, and therefore carcinoma has higher proliferating cell density than adenoma.