The incidence of primary lymphomas of the central nervous system (CNS) has significantly increased over the last years. However, the pathogenesis of this serious and fatal disease is still largely unknown. The aim of the present study was to investigate whether impairment of apoptosis is involved in the pathogenesis of primary CNS lymphomas. A series of 35 primary CNS lymphomas was investigated for the presence of apoptotic cells and the expression of apoptosis-inhibiting and proapoptotic gene products of the bcl family by application of the terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling (TUNEL) technique and immunohistochemistry. The majority (23/35) of the tumors contained no or less than 10% of apoptotic cells. All tumors were MIB-1 positive, and 53% of them showed a high proliferative activity with more than 20% MIB-1-positive cells. The bcl-2 gene was expressed in 54% of the tumors (19/35), whereas bcl-x and bax gene products were present in only a low fraction of these lymphomas (4/35). In contrast, bak and the tumor suppressor gene p53 product were not detectable. These findings indicate that apoptosis is inhibited in the majority of this series of primary CNS lymphomas. Since there was no statistical correlation between the degree of apoptosis and the expression of proteins of the bcl gene family, other apoptosis-inhibiting factors may be involved in the pathogenesis of primary CNS lymphomas.