In previously published studies on patients with juvenile chronic myelogenous leukemia (JCML), excessive proliferation of malignant monocyte-macrophage elements and impaired growth of normal hematopoietic progenitors were demonstrated. A selective hypersentivity of granulocyte-macrophage progenitors (CFU-GM) to granulocyte-macrophage colony stimulating factor (GM-CSF) seems to represent the main pathogenetic mechanism. Allogeneic bone marrow transplantation (BMT) has been demonstrated to be the only curative strategy for patients with JCML. In this study, we evaluated the growth of peripheral blood hematopoietic progenitors in semisolid cultures in two children with JCML before and after allogeneic BMT. Serum levels of GM-CSF, interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) were also assessed. IL-1-beta, GM-CSF and TNF-alpha serum levels of the patients before and after BMT did not differ significantly from those obtained in 45 healthy controls. After marrow transplant, the engraftment of donor hematopoietic stem cell was associated with the disappearance of both pretransplant GM-CSF hypersensitivity and CFU-GM spontaneous growth. The inhibitory effect on the growth of normal hematopoietic progenitors also resolved. This confirms that the substitution of the pathological hematopoietic progenitors represents the basis for the curvative effect of allogeneic BMT in the treatment of JCML, abolishing both the excessive responsiveness of JCML progenitor cells even to very low concentrations of GM-CSF and the growth-inhibitory effect on normal hematopoiesis.