The role of flow cytometry (FC) in the diagnosis of lymphoid lesions by fine-needle aspiration (FNA) is well established. However, studies evaluating the usefulness of FC in serous cavity effusions (SCE) are few. We performed a retrospective review of 115 consecutive SCE with concurrent FC analysis, comparing the provisional cytopathologic diagnosis (PCD), i.e., before the FC results were added, with final diagnoses as modified by subsequent FC immunophenotyping. The predominant clinical indication for the FC analysis was the presence of a spontaneous SCE in a patient with a history of malignant lymphoma. Three- or four-color analysis was performed using antibodies against CD45, CD71, CD33, CD22, CD19, CD20, κ, λ, CD5, CD3, and CD56. The PCD was benign in 47%, atypical in 16%, and malignant in 37% of cases. The latter category consisted mostly of malignant lymphoma (n = 32), but also included acute lymphoblastic leukemia (1 case), T-cell lymphoma/leukemia (2 cases), acute myelogenous leukemia (1 case), multiple myeloma (1 case), Hodgkin's lymphoma (1 case), sarcoma (1 case), and adenocarcinoma (4 cases). In 18 cases (16%), the PCD was later modified by the FC results from atypical/suspicious to benign (8) and from benign or atypical/suspicious to malignant (10 cases). The latter group included acute natural killer (NK) cell leukemia (1 case), chronic lymphocytic leukemia (1 case), mantle cell lymphoma (2 cases), follicular lymphoma (3 cases), angioimmunoblastic lymphoma (1 case), large cell lymphoma (1 case), and multiple myeloma (1 case). As expected, FC was noncontributory in cases of Hodgkin's lymphoma and nonlymphoid malignancies. In summary, immunophenotyping by FC modified the PCD significantly in 16% of SCE, permitting appropriate cancer staging and management. The above data underscore the importance of FC as an adjunct to cytomorphology in SCE. Diagn. Cytopathol. 2003;29:74–78. © 2003 Wiley-Liss, Inc.