We performed a retrospective clinicopathologic study on sequential biopsy specimens from 90 patients with Philadelphia chromosome-positive chronic myelogenous leukemia to study therapy-specific effects of busulfan (28 patients), hydroxyurea (32 patients), and interferon-alfa (IFN-alfa; 30 patients). Bone marrow specimens were evaluated by morphometry after silver impregnation and staining with monoclonal antibodies to identify reticulin fibers, nucleated erythroid precursors, megakaryocytes, and macrophages. To compute dynamics of histopathology implicating corresponding changes in time, relevant indices were calculated. Quantification of megakaryocytopoiesis and its precursor cell population showed a significant increase in the IFN-alfa and busulfan groups compared with the hydroxyurea group. These changes were associated with a development of myelofibrosis during therapy. Although a significant increase in fiber density was detectable in the busulfan group, the progression index proved to be twice as high after IFN-alfa therapy. In contrast, a considerable number of patients displayed a regression of myelofibrosis after hydroxyurea treatment. The general association of the megakaryocyte lineage with myelofibrosis was in line with experimental findings. The mature macrophage population and its activated subfraction revealed a marked proliferation (IFN-alfa group) during treatment. Growth and activation of macrophages may be compatible with their putative function during erythrocytopoietic regeneration and with stimulation of their phagocytic properties.