Inclusion Criteria:

        Inclusion criteria for trial entry and R1 randomisation.

          -  Diagnosis of acute myeloid leukaemia (AML) /high risk Myelodysplastic syndrome (MDS)
             (>10% blasts in the bone marrow)/isolated myeloid sarcoma (MS) (either de novo or
             secondary).

          -  Age <18 years.

          -  No prior chemotherapy or biological therapy for AML other than that permitted in the
             protocol.

          -  Normal cardiac function defined as fractional shortening ≥28% or ejection fraction
             ≥55%.

          -  Fit for protocol chemotherapy.

          -  Documented negative pregnancy test for female patients of childbearing potential.

          -  Patient agrees to use effective contraception (patients of child bearing potential).

          -  Written informed consent from the patient and/or parent/legal guardian.

        Inclusion criteria for participation in the gemtuzumab ozogamicin dose finding study:

        Centres must be formally activated in order to be take part in the embedded dose escalation
        study. Please contact the trial office for further information.

          -  Patient meets the inclusion criteria for trial entry.

          -  Age:

               -  ≥12 months for the major dose finding study

               -  ≥ 12 weeks and <12 months for the minor dose finding study

          -  Karnofsky or Lansky performance score of ≥50.

          -  Normal renal function defined as calculated creatinine clearance ≥90ml/min/1.73m2.

          -  Normal hepatic function defined as total bilirubin ≤2.5 upper limit of normal (ULN)
             for age unless it is caused by leukaemic involvement or Gilbert's syndrome or similar
             disorder.

          -  Alanine transaminase (ALT) or aspartate transaminase (AST) ≤10 x ULN for age.

          -  Written informed consent from the patient and/or parent/legal guardian.

        Inclusion criteria for participation in R2 (randomisation not yet open).

          -  Patient meets the inclusion criteria for trial entry

          -  Age ≥12 weeks.

          -  Karnofsky or Lansky performance score of ≥50.

          -  Normal renal function defined as calculated creatinine clearance ≥90ml/min/1.73m2.

          -  Normal hepatic function defined as total bilirubin ≤2.5 ULN for age and not due to
             leukaemic involvement or Gilbert's syndrome or similar disorder.

          -  ALT or AST ≤10 x ULN for age.

          -  Written informed consent from the patient and/or parent/legal guardian.

        Inclusion criteria for participation in R3.

          -  Patient meets the inclusion criteria for trial entry

          -  Induction treatment as per MyeChild 01 protocol or treated with 2 courses of
             mitoxantrone & cytarabine off trial.

          -  Minimal residual disease (MRD) response (performed in MyeChild 01 centralised
             laboratories, see national MyeChild 01 Laboratory Manual):

               -  Patients with good risk cytogenetics/molecular genetics and a MRD level <0.1% by
                  flow after course 2, or a decrease in transcript levels of >3 logs after course 2
                  for those with an informative molecular marker, but without an informative marker
                  of sufficient sensitivity for flow MRD monitoring or

               -  Patients with intermediate risk cytogenetics/molecular genetics with a MRD level
                  <0.1% by flow after course 1 and course 2, or a decrease in transcript levels of
                  >3 logs after course 1 and course 2 for those with an informative molecular
                  marker, but without an informative marker of sufficient sensitivity for flow MRD
                  monitoring.

          -  Written informed consent from the patient and/or parent/legal guardian.

        Inclusion criteria for participation in R4.

          -  Patient meets the inclusion criteria for trial entry

          -  Induction treatment as per MyeChild 01 protocol or treated with 1 or 2 courses of
             mitoxantrone & cytarabine ± treatment intensification with fludarabine, cytarabine &
             idarubicin (FLA-Ida) off trial.

          -  Patient is in complete remission (CR) or CR with incomplete blood count recovery (CRi)
             defined as <5% blasts confirmed by flow cytometry/ molecular/FISH in a bone marrow
             aspirate taken within 6 weeks prior to randomisation to R4.

          -  Patient meets one of the following criteria and is a candidate for HSCT as per the
             protocol:

               -  High risk after course 1 (all patients with poor risk cytogenetics and patients
                  with intermediate risk cytogenetics who fail to achieve CR/CRi).

               -  Intermediate risk cytogenetics with MRD >0.1% after course 1 and 2 measured by
                  flow. If no flow MRD marker of sufficient sensitivity is identified, a molecular
                  MRD marker with a sensitivity of >0.1% may be used.

               -  Good risk cytogenetics with flow MRD >0.1% confirmed by a decrease in molecular
                  MRD of <3 logs or rising transcript levels after course 3 despite treatment
                  intensification (FLA-Ida) and after discussion with the Clinical Co-ordinators.

          -  Availability of a 9-10/10 human leukocyte antigen (HLA) matched family or unrelated
             donor or 5-8/8 matched cord blood unit with an adequate cell dose as defined by the
             protocol section 17.1.

          -  Written informed consent from the patient and/or parent/legal guardian.

        Exclusion Criteria:

        Exclusion criteria for all randomisations

          -  Acute Promyelocytic Leukaemia.

          -  Myeloid Leukaemia of Down Syndrome.

          -  Blast crisis of chronic myeloid leukaemia.

          -  Relapsed or refractory AML.

          -  Bone marrow failure syndromes.

          -  Prior anthracycline exposure which would inhibit the delivery of study anthracyclines.

          -  Concurrent treatment or administration of any other experimental drug or with any
             other biological therapy for AML.

          -  Pregnant or lactating females.