The objective of this study was to determine the potential application of fine needle aspiration (FNA) and flow cytometry (FCM) for the diagnosis of non-Hodgkin's lymphoma (NHL). At our institution, we have used FNA coupled with FCM to accurately diagnose both reactive lymphoid processes and most lymphomas in a prospective series of more than 90 patients. A retrospective review of 266 patients with NHL who were treated at the London (Ont.) Regional Cancer Centre over a five-year period was performed. Two-thirds presented with palpable lymphadenopathy which would have been amenable to FNA. Based upon the results of our prospective study, FNA/FCM could have provided an accurate diagnosis of malignancy for these patients. Furthermore, the subclassification of the lymphomas would have been accurate in 75% of cases. For 25% of patients, including those with T-cell lymphoma, accurate subclassification may have been problematic. Fifty of the 266 patients developed recurrences during the period of study. Ninety per cent of these were at sites amenable to FNA. Furthermore, the cytomorphology could have indicated whether transformation to a higher grade of lymphoma had occurred. Currently, FNA coupled with FCM is an under-utilized diagnostic technique which has great potential for the investigation of patients with both newly diagnosed and recurrent lymphoma.