Background: The purpose of this study was to examine the expression of the cell adhesion molecule CD44 in normal lung, primary and metastatic lung tumors, and cell lines derived from primary lung carcinomas. Methods and Results: A total of 68 lung specimens including normal tissue and primary and metastatic tumors, as well as 28 cell lines cultured from primary lung tumors with high recurrence, were examined for CD44 expression by semiquantitative reverse transcription polymerase chain reaction. Variant exon expression was confirmed by Southern blotting and hybridization of particular samples. In tumor tissues, loss of CD44 variant expression correlated with increasing tumor stage; a smaller percentage of more aggressive and poorly differentiated tumors expressed CD44v. Tumors metastatic to the lung were negative for CD44 variant expression. In primary lung cell lines, as in tumor tissue, tumors of higher histologic grade were characterized by loss of CD44 variant expression. Conclusion: CD44 isoform expression in normal lung and tumor tissues and cell lines revealed an overall decrease in CD44 alternative splicing in lung neoplasms of increased malignancy.