During a period of four years (1981 to 1984), 641 ascitic, 860 pleural and 47 pericardial fluid specimens were examined cytologically. Of these, 154 ascitic samples, 174 pleural specimens and 10 pericardial effusions, obtained, respectively, from 108, 133 and 7 patients, were found to contain malignant cells. In 7 patients, ascites, and in 18 cases, pleural effusions were the first indication of cancer. None of the positive pericardial fluids was the initial presentation of malignancy. The cytologic findings and follow-up data on these 25 patients are the subject of this study. The most common type of neoplasm in these effusions was adenocarcinoma (86% of the ascitic and 78% of the pleural fluids). Most of the malignant neoplasms in ascitic fluids were derived from ovarian tumors (5 of 7) while those in pleural effusions came mainly from lung tumors (12 of 18). Mammary carcinoma, which was the most common malignant tumor found in cases of pleural effusions, did not present initially with an effusion in any of our cases. The cytologic diagnosis was confirmed in all cases by either biopsy or strong clinical evidence. The prognosis in patients who initially presented with an effusion was poor. All of the patients with an adequate follow-up died within 29 months in cases of ascites and within 19 months in cases of pleural effusions.