Levels of nm23-H1/nucleoside diphosphate kinase (NDP kinase) expression have been reported to correlate inversely with metastatic potential in some tumors but not in others. Whether or not nm23 gene product is associated with metastatic potential in lung cancer is not clear as yet. We therefore immunohistochemically examined the expression of nm23 gene products in primary lung adenocarcinomas according to cytologic subtypes in order to clarify the association of its expression with the clinical features of the disease. Seventy-two (64.9%) of the 111 lung adenocarcinomas were positive for nm23 protein. In lung adenocarcinoma of Clara cell type, high levels of nm23 expression were associated with advanced pathologic stage, positive lymph node status, and poorer prognosis (P < 0.05). However, no correlation with clinical outcome was observed in other cell types. Our data suggest that higher levels of nm23 expression are associated with tumor progression in lung adenocarcinoma of Clara cell type.