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Immunoglobulin (Ig) light chain restriction, monotypic expression of kappa or lambda light chains, is one of the hallmarks of B-cell non-Hodgkin lymphoma (B-NHL) and is readily detected using flow cyto-

Objectives Malignant effusion is usually caused by metastatic carcinoma. Malignant lymphoma is often not included as a top differential diagnosis of malignant effusion. Here, we describe 3 cases of young female patients with no significant past medical history who presented with fluid overload and were diagnosed with high-grade B-cell lymphoma (HGBL). Methods We conducted histopathologic examination and immunophenotypic and cytogenetic analyses on three cases using immunohistochemistry, flow cytometry, fluorescence in situ hybridization (FISH), and karyotyping. We also included patients’ clinical and radiological findings in our case reports. Results Histologic examination of the effusion samples showed numerous intermediate to large lymphoma cells with irregular nuclear contours and fine chromatin. The lymphoma cells were positive for CD10, CD20, BCL2, BCL6, and PAX5 and negative for CD34, cyclin D1, HHV-8, and TdT. In situ hybridization for Epstein-Barr virus (EBV)–encoded small RNAs was negative. The proliferation index by Ki-67 stain was more than 80%. Flow cytometry showed CD10-positive B cells with monotypic immunoglobulin light chain expression. Fluorescence in situ hybridization analysis demonstrated MYC, BCL2, or BCL6 rearrangements. These 3 patients were diagnosed as having HGBL with double-/triple-hit rearrangements. Despite receiving aggressive chemotherapy, all 3 patients had a dismal clinical course, with 2 patients dying less than 2 years after initial diagnosis. Conclusions High-grade B-cell lymphoma should be considered in the differential diagnoses of malignant effusions. Flow cytometric and FISH analyses of the body fluid specimens are essential to reach an accurate and timely diagnosis.