Expression of CD44 isoform that contains variant exons (v1-v10) has been implicated in tumor progression and metastasis. Especially, CD44 isoform containing v6 (CD44v6) and that containing v7 (CD44v7) were shown to confer full metastatic behavior on tumor cells. We examined the expression of CD44v6 and CD44v7 in malignant pleural effusions (13 lung cancers and 4 non-lung cancers) and in benign pleural effusions (7 tuberculosis and 3 pneumonia) with Southern blot analysis of reverse transcription (RT)-polymerase chain reaction (PCR) products. CD44v6 was expressed not only in malignant pleural effusions (12 of 13 lung cancers and 4 of 4 non-lung cancers), but also in benign pleural effusions(9/10). In contrast, although expression of CD44v7 was found in most malignant pleural effusions (12 of 13 lung cancers and 4 of 4 non-lung cancers), it was found in only a few cases of benign pleural effusions. These results suggest that the expression of CD44v7 may be correlated with a tumor-specific event such as metastasis or dissemination in malignant pleural effusion, while no such correlation can be found with CD44v6.