Twelve of 15 patients with juvenile chronic myelomonocytic leukemia (JCMML) referred to our unit underwent allogeneic bone marrow transplantation (BMT) between 1982 and 1992. BMT was not performed in the remaining three cases because of poor overall condition in two and disease progression in one. Six patients received marrow from HLA-identical siblings after a chemotherapy conditioning regimen in five cases. BMT failed in one case. Long-term remission was achieved in three patients and two others are in remission 6 and 11 months after BMT. Remission was associated with autologous recovery in one patient and minimal mixed chimerism in another. In one patient, a first BMT procedure resulted in autologous recovery and relapse. A second transplant, with chemotherapy conditioning including TBI, was successful. BMT with marrow from a matched unrelated donor was also successful. IN contrast, BMT with marrow from mismatched related donors (five patients) failed because of graft failure and/or relapse. This single-center series indicates that HLA-identical BMT is an appropriate treatment for JCMML. However, on the basis of these results it cannot be ascertained whether chemotherapy or splenectomy are necessary prior to BMT. The best chemotherapy conditioning regimen remains to be defined, as regimens consisting exclusively of chemotherapy resulted either in long-term remission or in autologous recovery with relapse.