The p53 tumor suppressor gene is mutated in the majority of pancreatic adenocarcinomas, and several studies have suggested that loss of p53 function may contribute to the aggressive clinical behavior of pancreas cancer. Although immunocytochemical accumulation of the p53 gene product has previously been assessed as a marker for p53 mutations in cancers of the pancreas and other organ systems, the relationship between p53 mutations and p53 protein accumulation is variable. The cyclin-dependent kinase inhibitor, p21 (also known as WAF1 and CIP1), is induced by wild-type but not mutant p53, and recent work has implicated p21 as a downstream mediator of the growth-suppressing and apoptosis-promoting functions of wild-type p53. In the present work, we sought to determine whether loss of p21 expression could more precisely identify those tumors with p53 mutations and/or loss, compared with immunocytochemical assessment of p53 protein accumulation. We evaluated p53 and p21 expression immunohistochemically in a series of 21 ductal adenocarcinomas of the pancreas with known p53 mutational status. Diffuse overexpression of p53 was found in 3 of 8 cases (38%) with wild-type p53 and 7 of 13 cases (54%) with p53 mutations with or without loss of heterozygosity at 17p. Surprisingly, expression of p21 correlated neither with p53 mutational status nor with p53 protein expression. In particular, strong p21 expression was seen even in carcinomas in which molecular analysis revealed a frameshift mutation in one allele of p53 and loss of the second. These data suggest that p21 expression in pancreatic adenocarcinoma may also be induced by a p53-independent pathway and that p21 expression, as assessed immunocytochemically, does not reflect the functional status of p53 in these carcinomas.