A total of 220 fine needle aspiration (FNA) specimens from 212 patients with clinically suspected or previously histologically confirmed lymphoma were evaluated by cytology in conjunction with immunophenotyping analysis of the aspirate; the results were compared with the histologic diagnosis made on previous or current accessions of lymph node or extranodal tissue. Smears of the aspirates were stained with the Diff-Quik and Papanicolaou stains while immunoperoxidase staining using antibodies against kappa and lambda immunoglobulin light chains and Leu-4 was routinely performed on Cytospin preparations. Where indicated, additional marker studies (including T-200, Leu-1, Leu-2a, Leu-3a + 3b, Leu-M1, B1, Leu-12, IgM, CALLA and TdT) were performed. For the non-Hodgkin's lymphomas, specimens were classified by the cytologic characteristics of the neoplastic cells according to the International Working Formulation scheme. The combination of cytologic smears and immunoperoxidase studies resulted in a diagnosis of lymphoma in 173 cases (79%). The remaining aspirates were interpreted as suspicious for lymphoma (7%), benign (10%) or inadequate for diagnosis (4%). Of the 15 suspicious aspirates, 5 proved to be Hodgkin's disease and 2 to be T-cell lymphoma by subsequent biopsy. The cause of failure in the nine inadequate aspirates were necrosis (3 cases), sclerosis (2 cases) and faulty technique (4 cases). In the cases that had concurrent tissue biopsies, no false-positive diagnoses were rendered. These results indicate that FNA used in association with immunocytochemistry is a reliable tool for establishing the diagnosis and classification of the majority of cases of lymphoma. Optimal immunoglobulin light-chain ratios for defining monoclonality in FNA specimens of B-cell lymphomas are proposed.