Objective:To study the diagnostic clues and significance in serous effusion cytology associated with lymphoblatic lymphoma/acute lymphoblastic leukemia (LBL/ALL).Methods:Forty-five serous effusion specimens with final diagnosis of LBL/ALL were collected from August 2011 to December 2019 at the First Affiliated Hospital of Zhengzhou University. All cases were reviewed for their clinical profiles, cytomorphologic features and ancillary studies. Cell blocks and immunocytochemistry were prepared in 22 cases; flow cytometric immunophenotyping was performed in three cases and gene rearrangement analysis (T-cell recepter, TCR and immunoglobulin, Ig) was performed in five cases.Results:Among the 45 cases, there were 35 males and 10 females with male to female ratio of 3.5∶1.0. The median age was 15 years. Mediastinal mass was the initial presentation in 39 patients (86.7%) and high LDL level were observed in 34 patients (75.6%). Microscopically, the majority of the specimens (86.7%) were hypercellular. The smears demonstrated dispersed lymphoblasts that were predominantly small to intermediate in size with scanty basophilic cytoplasm and irregular or convoluted nuclei with fine chromatin condensation and inconspicuous nucleoli. Mitoses were frequently observed. Karyorrhexis and apoptosis were seen in all cases. By immunophenotyping, TdT was expressed in 19 cases (86.4%) and CD99 in 20 cases (90.9%). Ki-67 expression varied from 65% to 95%. Flow cytometry in three cases demonstrated positivity for TdT, CD2, CD3 and CD7. Monoclonal TCR gene rearrangement was found in 4 of 5 cases, and both monoclonal TCR and Igκ gene were found in 1 case.Conclusions:In LBL/ALL, primary diagnosis could be made basing on clinical features (younger male patients with a mediastinum mass) and cytomorphology (monotonous, small to medium sized lymphoid cells with prominent irregular nuclei, fine chromatin and frequent mitoses, karyorrhexis and apoptosis). If immunocytochemistry and other ancillary studies are performed, the accuracy and reliability of the results could be improved.