Many articles concerning the accuracy of diagnosis of malignant tumor cells in body fluids have appeared in the literature, but few authors have attempted to describe the characteristics of these cells to determine the site of the primary tumor as they relate to tumors of specific primary sites. This paper presents the results of a retrospective study on malignant body cavity effusions of which the primary site was established on the basis of either biopsy or surgical resection of the primary neoplasm during the period of 6 years beginning from January 1983 to December 1988. The results obtained are summarized as follows: 1) The 143 fluid specimens from 129 patients were composed of 51 cases of pleural, 69 of peritoneal, and 9 of pericardial origin. 2) Adenocarcinoma was the most frequent type of malignant effusions (78.3%). The most common primary site was the lung (50%) in pleural fluid and stomach (55.2%) in ascites. 3) The results of this study show that the primary site of tumor cells can be identified in the body fluid of accurate cytomorphologic criteria are used. Identification of the primary site of an effusion would be improved by the consideration of clinical information