Summary When B‐cell lymphocytic leukaemia (B‐CLL) cells derived from peripheral blood were cultured in vitro, a substantial proportion of them spontaneously died by apoptosis. This type of cell death is morphologically and biochemically distinct from necrosis and has previously been found to occur under physiologic and certain pathologic conditions where cell deletion appears controlled and biologically meaningful. By 30 h of culture, approximately 20% of the unfractionated B‐CLL cells were affected. There was no significant difference in the incidence of apoptosis in T‐cell depleted and undepleted cultures or when either autologous or normal human serum was used. Furthermore, seeding densities of 2 × 106 and 5 × 105 cells/ml resulted in a similar incidence of apoptosis, indicating that cell density was unlikely to be a contributing factor in producing the death. The finding that B‐CLL cells spontaneously die in vitro has at least two important implications. Firstly, previous work relating to some of the functions of B‐CLL cells and their interactions with T cells may require re‐evaluation. Secondly, an understanding of the mechanisms involved in the induction of apoptosis in this disease may have therapeutic consequences.