CML is a rare malignancy in childhood not yet being treated by an established protocol of the Society of Pediatric Oncology and Hematology (GPOH). Up to now only allogeneic bone marrow transplantation (BMT) offers a curative treatment option. After being enrolled into the study "CML-päd" patients with a matched sibling donor should undergo BMT within six months after diagnosis and within one year if an unrelated donor can be identified. Progress in palliative therapy results from the experience in adult patients: Hydroxyurea in combination with interferon alpha (IFN) is the treatment of choice. One aim of the study is to monitor quantitatively in bone marrow aspirates the activity of the disease in all patients on treatment every three months by performing cytogenetics and molecular analysis. As a pilot protocol intensification of treatment by idarubicine and cytarabine is scheduled one year after diagnosis for those patients not undergoing BMT and exhibiting after IFN treatment less than partial response. After progress to the accelerated phase of the disease a decision has to be made on an individual basis whether the patient should undergo new experimental treatment approaches.