AIMS: To assess the value of flow cytometry (FCM) in the diagnosis and classification of reactive lymphoid hyperplasia and malignant lymphoma by fine needle aspiration (FNA) cytology. METHODS: Forty six fine needle aspirates of lymphoproliferative disorders were examined by FCM as well as routine cytological assessment. An immunoglobulin light chain ratio (LCR) was calculated for clonality analysis. Additional immunophenotyping was performed in 15 cases. RESULTS: All 25 cases of reactive lymphoid hyperplasia were polyclonal by FCM (LCR < 2/1); 17 of 20 cases of B cell non-Hodgkin's lymphoma were monoclonal (LCR > 3/1). Analysis of cells based on size facilitated detection of small populations of clonal neoplastic cells. Analysis of CD5, CD10, and CD23 expression by FCM facilitated subclassification of mantle cell lymphoma, small lymphocytic lymphoma, and some lymphomas of follicle centre cell origin. One case of T cell non-Hodgkin's lymphoma was correctly classified by FCM. CONCLUSIONS: FNA cytology is a reliable method for investigation of lymphoproliferative disorders. Although excision biopsy and histopathological examination remain the gold standard for primary diagnosis and classification of non-Hodgkin's lymphoma, FNA cytology with clonality analysis and immunophenotyping by FCM is useful for distinguishing reactive from neoplastic lymphoid populations, and can facilitate lymphoma classification.