We studied 48 non-Hodgkin lymphoma (NHL) fine-needle aspiration (FNA) specimens with initial cytomorphology (CM) and flow cytometry (FC) and subsequent surgical biopsy of the same lesion to determine whether a reliable diagnosis of large cell lymphoma or large cell transformation could be made. CM was evaluated by examining 200 lymphocytes in each specimen. FC was performed by analyzing monoclonal or abnormal B-cell populations. Percentages of large cells were evaluated by CM and FC and results correlated with the histologic diagnosis. All small cell NHLs showed fewer than 40% large cells by CM and FC; 100% (9/9; FC) and 67% (6/9; CM) of diffuse large B-cell lymphomas demonstrated greater than 40% large cells. Variable numbers of large cells were detected in grade III follicular lymphoma, low-grade lymphoma with partial large cell transformation, and large B-cell lymphoma containing fewer than 10% neoplastic cells. By using combined CM and FC, large cell lymphoma and large cell transformation can be diagnosed reliably by FNA if greater than 40% large cells are present. Surgical biopsy is necessary when there is necrosis, fewer than 10% neoplastic cells by FC, or fewer than 40% large cells with clinical signs of transformation.