p21 (p21WAF1/Cip1), a cyclin-dependent kinase inhibitor, induces G1 arrest and can inhibit the activity of the proliferating cell nuclear antigen (PCNA). We analyzed p21 expression during colorectal tumorigenesis, its association with its transcriptional regulator p53, and its relationship to rates of cell proliferation and apoptosis. p21 and p53 protein expression were examined in sporadic tumors and hereditary nonpolyposis colorectal cancers (HNPCCs) by immunohistochemistry (IHC) and immunoblotting. Apoptosis was examined using a DNA nick end-labeling assay, and cell proliferation was examined by PCNA staining. In normal colorectal epithelia, nuclear p21 staining was uniformly detected in crypt cells of the superficial compartment (upper one-third) that stained negatively for PCNA. p21 and PCNA expression were, therefore, mutually exclusive. In sporadic cases, a decrease in the frequency of p21 expression accompanied adenoma development and progression to carcinoma. Specifically, p21 was detected in 12 of 16 (75%) adenomas and 10 of 32 (31%) carcinomas. In contrast to sporadic cases, HNPCCs with known mutations in DNA mismatch repair genes expressed p21 in 12 of 15 (80%) carcinomas. An inverse relationship between p21 and p53 was observed wherein mutant p53 proteins were detected in 4 of 15 (27%) HNPCCs versus 22 of 32 (69%) sporadic carcinomas. Although p21+ carcinoma cells were generally negative for p53, IHC revealed that some carcinoma cells expressed both p21 and p53 proteins. Furthermore, p53-mutated SW480 colon carcinoma cells were found to coexpress p21 and p53, suggesting that p21 can also be activated by a p53-independent mechanism. No association was found between p21 or PCNA and apoptotic labeling indices in adenomas or carcinomas. In conclusion, a decrease in p21 expression accompanies neoplastic progression in sporadic cases but not in HNPCCs. This finding appears related to p53 status in that the frequency of p53 expression was significantly reduced in HNPCCs compared to sporadic cases, suggesting a difference in their molecular pathways of tumorigenesis.