Chronic leukemias of early childhood (JCML/CMML) are rare malignant diseases for which effective regimens of chemotherapy have not been established. However, some of these patients may be cured by BMT. We report on 4 children with JCML and one child with CMML undergoing BMT from HLA-identical siblings and from matched unrelated donors. 3 of 5 patients are disease-free 9 to 37 months post BMT. According to our observations an effective reduction of blastic cells before BMT seems to be necessary for a sustained remission of these diseases. This reduction can be reached by intensive chemotherapy and by splenectomy before BMT. Moreover, we stress the need for total body irradiation (TBI) during BMT in order to eradicate residual malignant cells.