To study the biopathological characteristics of the transitional mucosa adjacent to rectal carcinoma, 34 cases were subjected to mucin histochemical and immunohistochemical study to observe the expression of p53 and p21 protein in distal mucosa adjacent to rectal carcinoma and its relationship to the mucin change. The expression of p53 protein was found in 29.4% (10/34) of distal transitional mucosa in the cytoplasm of goblet cells, and its positive staining was within 4 cm from carcinoma margin. All p53 positive mucosa was transitional mucosa. Overexpression of p21 protein was found in 26.5% (9/34) of distal transitional mucosa in cytoplasm of crypt cells, and its positive staining was within 2 cm from carcinoma margin. There was no relationship between the expression of p53 and p21 protein in carcinoma and that in transitional mucosa (P > 0.05). These findings indicated that there was aberrant alteration of p53 and p21 genes in transitional mucosa adjacent to colorectal carcinoma, which provided further evidence that transitional mucosa was an unstable pre-cancerous change. The aberrant mucin change and genetic alteration in distal mucosa of rectal cancer is within 4 cm.