Studying Conditioning Regimen In Pediatric Transplantation - AML , SCRIPT-AML

Inclusion criteria for randomization part of the study: * Age ≤18 years at time of initial AML, age ≤ 21 years at transplantation. * HCT is performed in a study participating center * All women of childbearing potential who have to have a negative pregnancy test within 2 weeks prior to the start of treatment. * Signed informed consent. * Any relapsed AML after initial treatment according to a defined international AML protocol. (NOPHO-DBH AML 2012/new protocol), or AML in first remission with transplant indications and treatment according to national AML protocol (NOPHO-DBH AML 2012 or new protocol). * In hematological remission, defined as: \< 5 % leukemic blasts confirmed by flow cytometry (in patients with an informative leukemia associated immunophenotype) in a bone marrow sample taken ≤14 days prior to start of conditioning and no evidence of extramedullary disease, including in CNS and no leukemic blasts in the peripheral blood (verified by flow cytometry in case immature cells are detected in the peripheral blood differential). -Patients must have a related or unrelated donor fulfilling any of the following criteria: HLA 10/10 allelic matched, identical, sibling BM donor or HLA 10/10 or 9/10 allelic matched related/unrelated BM or PBSC donor orHLA 5-6/6 unrelated or 6-7-8/8 unrelated Cord Blood (UCB) Inclusion criteria for observation/registration only: * Diagnosis of acute myeloid leukemia * Indication for allogeneic stem cell transplantation, as defined by primary treatment protocol or treating physician. * Age ≤18 years at time of initial AML, age ≤ 21 years at transplantation. * Not eligible for randomization, either due to lack of consent or not fulfilling inclusion criteria for interventional part of the study. * Signed informed consent to prospectively register follow-up data. Exclusion criteria for the randomization part of the study : * Diagnosis of myelodysplastic syndrome (MDS). * Diagnosis of juvenile myelomonocytic leukemia (JMML). * History of previous malignancy (AML diagnosed as secondary cancer). * Known diagnosis of Fanconi anemia. * Prior autologous or allogeneic hematopoietic stem cell transplant. * Planned prophylactic DLI or other immunotherapeutic interventions after HCT that are not included in the upfront protocol, Planned anti-leukemic medication after HCT that are not included in the upfront protocol * Known intolerance to any of the chemotherapeutic drugs in the protocol. * Major organ failure precluding administration of planned chemotherapy. * Patients with uncontrolled bacterial, viral, or fungal infections (currently taking medication and with progression or no clinical improvement) at time of enrollment. * Severe concomitant disease that does not allow treatment according to the protocol at the investigator's discretion, e.g. malformation syndromes, cardiac malformations, metabolic disorders, renal impairment (\<30% of normal glomerular filtration rate), severe pulmonary, hepatic or cardiac impairment due to toxicity or infection. * Karnofsky / Lansky score \< 50% * Females who are pregnant (positive serum or urine βHCG) or breastfeeding. * Females of childbearing potential or men who have sexual contact with females of childbearing potential unwilling to use effective forms of birth control or abstinence for one year after transplantation. * Subjects unwilling or unable to comply with the study procedures. Exclusion criteria for the observational part of the study: * Diagnosis of Myelodysplastic syndrome (MDS). * Diagnosis of Juvenile myelomonocytic leukemia (JMML). * Age above 21 years at time of transplantation * No consent is given to prospectively register outcome data * Prior autologous or allogeneic hematopoietic stem cell transplant.