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Cytology of serous effusions is an important diagnostic tool for the diagnosis of cancer, staging, and prognosis of the patient. Herein, we retrospectively applied the International System for Reporting Serous Fluid Cytopathology (TIS) and provided the corresponding risks of malignancy (ROMs).Pleural, pericardial, and peritoneal effusion samples were retrieved from the archives of our department and reclassified according to the TIS. The ROM for each category was calculated based on available surgical follow-up.A total of 3790 effusions were studied. Pleural samples (1292) were reclassified as follows: 27 (2.1%) as non-diagnostic (ND), 1014 (78.5%) as negative for malignancy (NFM), 86 (6.6%) as atypia of undetermined significance (AUS), 29 (2.3%) as suspicious of malignancy (SFM), and 136 (10.5%) as malignant (M). Pericardial samples (241) were reclassified as follows: 4 (1.6%) as ND, 173 (71.8%) as NFM, 10 (4.1%) as AUS, 7 (3%) as SFM, and 47 (19.5%), as M. Peritoneal cases (2257) were re-categorised as follows: 31 (1.4%) as ND, 1897 (84%) as NFM, 39 (1.7%) as AUS, 53 (2.4%) as SFM, and 237 (10.5%) as M. The respective ROM values for each category were 18.5%, 15%, 45.3%, 93%, and 100% in pleural effusions; 25%, 13.2%, 35%, 100%, and 100% in pericardial effusions; and 19.3%, 10.4%, 43.5%, 100%, and 100% in peritoneal effusions.Pleural, pericardial, and peritoneal cytology show high specificity and moderate sensitivity in the evaluation of serous effusions. The ROMs reported in our study were mostly concordant with those published according to the TIS.