The immunoreactivity of p21 was evaluated in normal mucosa and in adenomas and adenocarcinomas of the human large bowel. In normal mucosa, p21 immunoreactivity was seen in the superficial third of the crypts (maturation compartment) and in surface (terminally differentiated) epithelium, and was mutually exclusive with Ki67/MIB1 reactivity. These data show that p21 expression is inversely related to proliferation and directly related to terminal differentiation. In adenomas, p21-reactive cells were frequently clustered in the superficial areas and were non-reactive for MIB1. In adenocarcinomas, p21 staining was heterogeneous: high p21 expression (19 cases) was associated with lower stage and lack of p53 overexpression. p21-reactive cells were devoid of MIB1 immunoreactivity, but no relationship could be found between p21 and MIB1 labelling indices. p21 heterogeneity may be related to alterations in the p53-dependent induction pathway: high p21 expression was associated with low to absent p53 reactivity, with presumed normal p53 function; low p21 expression was associated with p53 overexpression, with presumed p53 alteration resulting in loss of function.