Aim: Evaluation of the usefulness of flow cytometry (FCM) serous effusion analysis in a diagnosis of pediatric Non-Hodgkin lymphomas (NHL). Introduction: Serous effusions are often the first, life-threatening manifestation of pediatric NHL. FCM immunophenotyping of effusions with cytological analysis could help in diagnosis of NHL, and thus enable fast initiating of cytoreductive therapy. Material and Methods: FCM analysis of serous effusions obtained from 17 children and adolescents hospitalized in Mother and Child Healthcare Institute of Serbia under clinical suspicion of NHL using the standardized panel of monoclonal antibodies: CD19, iCD79a, CD20, CD10, iIgM, kappa/lambda, iCD3, sCD3, CD7, CD2, CD5, CD4, CD8, CD1a. Cytological examination was performed on May-Grunwald-Giemsa stained slides. The results were correlated with histopathological findings of available tumor biopsies. Results: Precursor T-cell (T-III/T-IV) phenotype was confirmed in 5 samples. In 7/9 samples with mature B (B-IV) phenotype, FAB L3 cytomorphology indicated Burkitt lymphoma (BL), and in 2/8 suggested diffuse large B-cell lymphoma (DLBCL). Tumor biopsy was available in 7/14 patients and in all cases preliminary diagnosis was confirmed. In 3 patients with no malignant cells in effusions, FCM and cytomorphologicaly only reactive changes were observed, and diagnosis had to be made by tumor biopsy (BL 2 patients, DLBCL 1 patient). Out of 7 patients diagnosed only by FCM and cytological analysis, 6 achieved a remission of the illness. Conclusion: FCM detects NHL cells in malignant serous effusions fast and accurate. In combination with cytological analysis, FCM is sufficient for diagnosis in most cases, allowing rapid initiation of therapy.