The primary objective of this study is to evaluate the safety of gemtuzumab ozogamicin in relapsed CD33-positive AML patients who received HSCT. If the MTD dose is not reached, 9 mg/m2 will be the maximum tested dose. A secondary objective is to assess efficacy in terms of the number of patients attaining a complete (CR) or morphological (CRp) remission.

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.