The expression of the p53-inducible cyclin-dependent kinase inhibitor p21WAF1/CIP1 in non-neoplastic mucosa, adenoma, and adenocarcinoma of the stomach was examined immunohistochemically and its relationship with p53 expression and proliferative activity was analysed. In normal gastric mucosa as well as in intestinal metaplasia the epithelial cells at the surface which showed no proliferative activity expressed p21WAF1/CIP1, whereas the cells in the deep area of the glands expressing Ki-67 did not. In the neoplastic lesions, the expression of p21WAF1/CIP1 was detected in 78 per cent (112/144) of the adenomas and 76 per cent (262/343) of the adenocarcinomas. The incidence of p21WAF1/CIP1 expression did not differ among histological types of gastric carcinoma. The strong expression of p21WAF1/CIP1 was more frequently observed in carcinomas invading into submucosa or in cases of stages 2, 3, and 4 than in carcinomas limited to the mucosa or in stage 1 cases. The incidence of strongly positive cases was higher in carcinomas with lymph node metastasis than in those without metastasis. There was no apparent correlation between the expression of p21WAF1/CIP1 and the abnormal accumulation of p53 or with proliferative activity measured by Ki-67 expression. These findings overall suggest that p21WAF1/CIP1 might be associated with the senescence of non-neoplastic gastric epithelial cells; that a p53-independent pathway might be substantially involved in the induction of p21WAF1/CIP1 in gastric neoplasia; and that the proliferative activity of gastric cancer might not be solely dependent on control of the cell cycle by p21WAF1/CIP1.