Adenocarcinomas are the most common cause of malignancy in pleura fluids. Usual primary sites include the lung, breast, gastrointestinal tract, and genitourinary tracts. Predicting the site of origin of an adenocarcinoma can be difficult due to overlapping morphologic characteristics. We investigated the use of alkaline phosphatase (AP), Cytokeratin7 (CK7) Cytokeratin8 (CK8) to distinguish adenocarcinomas of lung in 148 body cavity fluid samples. Overall results for primary lung adenocarcinomas, demonstrated CK8 reactivity in 106 (72%) of 148 cases. 95 primary lung carcinoma samples (65%) were positive for CK7. AP was expressed in 81% of primary lung adenocarcinomas. Positive immunoreactivity for AP was characterized by a red, diffusely apical cytoplasmic staining in tumor cells that ocurred singly or in groups. There was a significant difference between AP, CK 7 and CK 8 expressions in primary lung adenocarcinomas (P=0.02; Chisquared test). The sensitivity of AP, CK8, CK7 as a marker for primary lung adenocarcinomas were 82%, 72%, 64%, respectively. Thus the AP positive staining largely confirmed the cytologic diagnosis of lung adenocarcinoma.