Seven children (age range 1.8–11 years) with juvenile chronic myelomonocytic leukaemia (JCML) received an allogeneic bone marrow transplantation (BMT), four from an HLA-identical sibling and three from a matched unrelated donor. In the four children transplanted using an HLA-identical sibling, conditioning regimen included busulfan (BU), cyclophosphamide (CY) and melphalan (L-PAM), whereas graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine-A (Cs-A). The preparative regimen was well tolerated and all patients engrafted promptly. None of the patients have relapsed and all four children remain in haematological remission after an observation time of 7, 24, 25 and 48 months, respectively. Of the three children given BMT from an unrelated volunteer, one was <2 years of age and she received the BU/CY/L-PAM regimen. In view of the increased risk of graft rejection described in patients transplanted from unrelated donors, we chose to prepare the other two patients with fractionated total body irradiation (TBI), thiotepa and CY. Cs-A, short-term methotrexate and Campath-1G in vivo were employed to prevent GVHD in this group of patients. Graft failure with autologous reconstitution of haemopoiesis occurred in the child given the chemotherapy-based regimen. One of the two girls given TBI relapsed after BMT; therefore only one of the three patients who received a marrow transplant from a matched unrelated donor survives in complete haematological remission 10 months after BMT. Our study suggests that the conditioning regimen we employed for allogeneic BMT from a compatible sibling is an effective means of eradicating the leukaemic clone. In our experience, results obtained using unrelated donors are less satisfactory and, at present, the use of such donors seems to be riskier and associated with a lower success rate as compared with BMT from an HLA-identical sibling.