Inclusion Criteria

Inclusion Criteria

- Patients with CD33+ AML who have experienced relapse after autologous or allogeneic
HSCT. Flow cytometry, performed at the study site, must demonstrate that the patient
has AML that is CD33 positive, based on local laboratory criteria (Patients with
history of MDS pretransplant will be eligible if their proportion of myeloblasts by
marrow aspirate exceeds 5% at time of evaluation for Gemtuzumab Ozogamicin treatment)

- The patient must be greater than or equal to 60 days post-HCST

- Patients of all ages may be entered in this study

Exclusion Criteria

- Peripheral white blood count (WBC) greater than or equal to 30,000/uL at the time of
initial Gemtuzumab Ozogamicin administration. (In order to reduce the peripheral
blood count, patients may be treated with Hydroxyurea. Hydroxyurea must be
discontinued 24 hours prior to Gemtuzumab Ozogamicin administration.)

- Known active central nervous system (CNS) or testicular leukemia at time of study
entry.

- Prior therapy with anti-CD33 antibodies.