Background : Both non-malignant and malignant causes of effusion can be identified by the relatively non-invasive technique of pleural fluid cytology. With this basis the present study on cytology of pleural fluids was taken up. The diagnostic significance of the cytologic study of the fluid may be attributable to the fact that the cell population present in the sediment is representative of a much larger surface area than that obtained by needle biopsy. Materials and Methods : One hundred samples of pleural fluid were examined for total cell count, cell type and cellular features. They were also subjected to biochemical study to find out the level of protein, glucose and chloride. Results : A total of 82% samples were exudative and 18% were transudative. Total leukocyte count (TLC) was less than 1000 cells/cu.mm in most (88.89%) of transudative effusions. Overall 52.44% of exudative effusions had TLC greater than 1000 cells/cu.mm. It was noted that 96.88% of tuberculous effusions had more than 50% lymphocytes, 81.25% had protein greater than 5 gm/dl and 90.63% had glucose greater than 60 mg/dl. Approximately 28% of pleural effusions were positive for malignant cells. Most (82%) of malignant effusions were exudative. The primary site could be assessed by cytological examination in 57.14% of malignant effusions. Conclusions: The most useful test in establishing the diagnosis of pleural effusion is pleural fluid cytology and pleural fluid cell count. Cytologic study of pleural fluid is a complete diagnostic modality which aims at pointing out the etiology of effusion as well as, in certain cases, a means of prognostication of disease process.