A comparative morphometric analysis was performed on smears and trephine biopsies of normal bone marrow and in chronic myelogenous leukaemia (CML) to assess the effects of therapy on apoptosis and cell proliferation. The in situ end-labelling (ISEL) technique was used for the demonstration of programmed cell death, in combination with the monoclonal antibody PG-M1 to identify macrophages. Cell proliferation was evaluated by employing the monoclonal antibody PC10 directed against proliferating cell nuclear antigen (PCNA). In CML (48 patients), significantly higher rates of apoptosis were observed than in normal bone marrow (smears, frozen sections, and paraffin-embedded samples) of 15 patients. In contrast, the PCNA labelling index of CML was not different from controls. In bone marrow tissue derived from CML patients, about 36 per cent of apoptotic bodies were ingested with CD68-positive macrophages. Study of the histotopographical distribution of labelled cells revealed that in CML, in contrast to the normal bone marrow, programmed cell death and PCNA activity were concentrated along the paratrabecular generation zone. In 28 patients with CML treated with interferon (IFN), sequential trephine biopsies displayed a significant enhancement of apoptosis which was associated with a decrease in PCNA reactivity. In contrast to this finding, no such alterations could be observed in 24 patients who received busulfan (BU) monotherapy. This study furthers the understanding of cell kinetics in CML. IFN therapy induces apoptosis and suppresses cell proliferation. The rate of programmed cell death prior to therapy and the extent of IFN-triggered apoptosis exert a significant predictive impact on survival. In this study, ISEL-positive (apoptotic) cells and bodies do not correspond to unscheduled cell repair as detected by PCNA immunoreactivity.