The fraction of proliferation cells was analysed in fine needle aspirates from a series of 448 non-Hodgkin's lymphomas and 199 reactive hyperplasias using an immunoperoxidase staining with monoclonal antibody Ki-67. There was a good correlation between proliferation fraction and cytologic assignment to high and low grade lymphomas. Thus high grade lymphomas had a high median percentage of Ki-67 positive cells with a figure of 82.1 for lymphoblastic, 60.0 for immunoblastic, and 59.7 for centroblastic lymphomas. For low grade lymphomas the figures were 17.1 and 11.1 percent for centroblastic/centrocytic and CLL/immunocytoma, respectively. The fraction of proliferation cells in reactive lymphadenitis varied between 1-50% with a median of 11.5%. Analysis of Ki-67 positivity can accordingly not be used to differentiate benign from neoplastic proliferations. Within all lymphoma subgroups but lymphoblastic lymphoma, there was a marked variation in fraction of Ki-67 positive cells, which resulted in a certain overlap between high and low grade lymphomas. The results show that cells procured through fine-needle aspiration can be used to analyse the fraction of proliferating cells which contributes information about the growth rate of the individual tumours that can not be obtained through cytologic classification.