No established criteria exist for predicting lymphoma grade or transformation in cytologic material. We counted transformed lymphocytes in fine-needle aspiration (FNA) biopsy specimens to determine whether the percentage of these cells in the smear could predict the histologic grade, the biologic behavior, or both. The percentage of transformed lymphocytes out of total lymphoid cells was determined on Papanicolaou-stained smears. Afterward, a cytodiagnosis was based on clinical information available at the time of the FNA, cytomorphologic data, and flow cytometry data. Results were correlated with results of examination of the surgical biopsy specimen, clinical behavior of the lymphoma, or both. The percentage of transformed lymphocytes was 10% or less in all low-grade or indolent lymphomas. Aspirates with transformed lymphocyte counts of 20% or greater were aggressive lymphomas. We also report our experience in the diagnosis of non-Hodgkin's lymphoma by FNA using cytomorphologic examination and immunophenotyping by flow cytometry at a cancer referral hospital. This is a preliminary study, and larger series may help establish the ranges of transformed lymphocyte counts that correlate with the lymphoma subtype.