- 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)