To elucidate the role of CDK inhibitor p21WAF1/CIP1 in human oesophageal squamous cell carcinomas, we examined its expression immunohistochemically using surgically resected tissues from 25 patients, and have analyzed the relationship with alteration of p53 gene (F-SSCP analysis), proliferative activity (Ki-67 labelling index), frequency of apoptosis (in situ DNA nick end labelling), and degree of differentiation. P21 expression was observed in 11 cases (44%) with a percentage of positive cells ranging between 1% and 10%. Of the 25 cases, 4 cases showed > 5% of positive cells. As for the relationship with p53 gene, all 7 p53-mutation positive cases were negative for p21 expression, whereas 11 out of 18 mutation negative cases showed positive for p21 expression. As for the relationship with degree of tumour differentiation, 6 out of 8 well differentiated type cases showed positive for p21 expression. By contrast, all 8 cases of poorly differentiated type were negative for p21 expression. Frequency of apoptotic cells was significantly higher in p21 positive cases than negative cases although Ki-67 labelling index was almost the same regardless of the expression of p21. P21 expressing cells were distributed mainly in the middle layers of the invading nests, especially around the keratinization, which was almost similar to the distribution of apoptotic cells. Our results suggest that expression of p21 in human oesophageal squamous cell carcinomas is induced by a p53-dependent pathway and affects apoptosis and differentiation of carcinoma cells.