Malignant effusions, such as pleural, pericardial, or peritoneal ones, are extremely uncommon in leukemia, and research on serous effusions has predicted that less than 1% of all effusions will have leukemia involved. The clinical care of patients with a hematologic illness relies on accurate cytological distinction between reactive and malignant effusions. Malignant effusions' prognostic significance has been widely debated in the literature. We present a case of a 33-year-old woman who had complained of having a fever and having shortness of breath for a month. An aspiration of the bone marrow revealed that blast cells had almost completely replaced the bone marrow. A cytopathological investigation of the pleural fluid was conducted after it was transferred to the pathology department. Cytological characteristics demonstrate leukemic infiltration. On a sample of bone marrow aspirate, further immunophenotyping was conducted. The diagnosis of T-cell acute lymphoblastic leukemia was made based on the results of the IPT.