Expression of p53, a tumor-suppressor gene product, was studied immunohistochemically in microwave-fixed, paraffin-embedded sections of 84 colorectal carcinomas and 44 adenomas. Using a monoclonal antibody (PAb1801), nuclear p53 was detected successfully in 51 of 84 (60.7%) cases of carcinomas, and no stain for p53 was demonstrated in the adjacent normal mucosa. The results in the microwave-fixed, paraffin-embedded sections correlated with those in the frozen sections. The incidence of p53 expression in colorectal carcinomas was high in the cases with distant metastasis, but it was not affected by clinicopathologic features such as tumor size or depth of invasion. In colorectal adenomas, only 4 of 44 (9%) adenomas expressed p53. This expression of p53, however, was restricted to only a few glands within tubular adenomas with mild dysplasia. Thus, p53 protein was expressed preferably by malignant tumors of the colorectum. The present study demonstrated the usefulness of microwave fixation to preserve p53. The immunohistochemical detection of p53 in microwave-fixed, paraffin-embedded sections of colorectal carcinoma and adenoma can provide valuable information about the mechanism of carcinogenesis in colorectal epithelium.