This paper reports on a comparative immunohistochemical study of p53 and proliferating-cell nuclear antigen (PCNA) performed on microwave (MW)-fixed paraffin sections of 24 colorectal carcinomas and 68 adenomas in order to investigate the relationship between p53 expression and proliferative activity. Nuclear p53 was detected in 12 cases out of 24 carcinomas and eight out of 68 adenomas were found to focally express p53. Examination of PCNA expression in the adjacent sections of carcinomas revealed that the incidence of PCNA expression was not affected by p53, and both p53-positive and p53-negative carcinomas exhibited a high level of PCNA expression that reached more than 75% of the cancer cells. In the colorectal adenomas, the incidence of PCNA expression averaged about 30% although highly heterogenous distribution of PCNA-positive cells in the section was observed. When we examined eight adenomas positive for p53, the incidence of PCNA expression in the p53-positive glands of each adenoma was significantly higher (52.1%) than that in the neighboring glands without p53 expression (36.5%) (p<0.05). The results indicate that the abnormal expression of p53 in colorectal adenomas may reflect a failure in the control of cell proliferation. Furthermore, lack of the association of p53 expression with proliferative activity in colorectal carcinomas implies that several gene alterations together with p53 may be responsible for the complete loss of the regulatory mechanism for cell proliferation.