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Malignant pleural effusions (MPE) are common complications in cancer patients and indicate the spread of the primary disease (metastasis). For over a century, malignant effusion has been diagnosed through the observation of changes in the cavity effusion cells, such as the gold-standard cytomorphological analysis. Studies show that the multiparameter flow cytometric analysis is sensitive and rapid and allows for the immunophenotypic evaluation of a large number of cells. This report describes the case of a 64-year-old man diagnosed with lung cancer who had never had any kind of treatment for this disease. The pleural sample was analyzed by morphological (quantitative and differential cytology) and immunocytochemical analyses. As a new tool for diagnosis of malignant pleural effusion, the sample was also analyzed by flow cytometry. In the case report described, flow cytometry was an effective and quick method for detecting neoplastic cells in pleural fluids.