To further investigate whether multiple genetic changes are involved in the development of colorectal cancer, we performed an immunohistochemical analysis of p53 and ras p21 protein expression in 139 specimens of colorectal adenoma with varying degrees of dysplasia, 57 specimens of early cancer with an adenomatous component, and 12 specimens of superficial early cancer without any adenomatous component. Positive p53 staining was found in 15% of the adenomas with moderate dysplasia and in 42% of the adenomas with severe dysplasia or intramucosal carcinoma (IMCA). Positive immunostaining of p53 was observed to be significantly correlated with the degree of dysplasia and the depth of invasion, as was the expression of ras p21. However, a closer correlation was observed with the increasing size of the adenomas. Furthermore, p53 staining was positive in 42% of the 12 superficial early cancer specimens, while ras staining was positive in only 1 specimen (8%). These results indicate that p53 gene overexpression may play some biological role in both the adenoma-to-carcinoma sequence and in de novo cancer development, whereas ras p21 expression may not be as involved in de novo cancer development as in the malignant conversion of colorectal adenomas.