p21WAF1/CIP1 protein is a cyclin-dependent kinase inhibitor, discovered to be a downstream effector of p53-dependent cell cycle regulation. In order to elucidate the significance of p21 expression in lung adenocarcinomas, we performed immunohistochemical analysis of p21, p53 and Ki-67 expression in surgically resected lung adenocarcinomas. In non-neoplastic tissue, a few bronchial and bronchiolar suprabasal and ciliated cells, and a few type II alveolar cells and alveolar macrophages in the peripheral lung, were p21 positive, but the positive rate in normal lung tissue was very low (<1%). All 91 lung adenocarcinomas examined showed p21 immunoreactivity: 39 cases (42.9%) and 52 cases (57.1%) showed high and low p21 expression levels, respectively. There was no significant correlation between p21 expression and p53 expression, the loss of heterozygosity status of the p53 gene, histological grade determined by the predominant histology, lymph node metastasis, pathological stage, tumor size, smoking history or gender. A positive, not inverse, correlation between p21 and Ki-67 expression was observed. We also observed heterogeneous expression of p21 in lung adenocarcinomas, i.e. in about two-thirds of the tumors, the tumor cells in the peripheral regions were p21 positive more frequently than were those in the central regions. More intense p21 expression tended to occur in the more highly differentiated areas. These results suggest that p21 is involved in tumor cell differentiation and the physiological mechanism that protects against tumor extension.