Gemtuzumab ozogamicin (GO) is a humanized anti-CD33 monoclonal antibody conjugated to a derivative of an antitumor antibiotic, calicheamicin. GO was approved for the treatment of relapsed acute myeloid leukemia (AML) in the United States (US) in 2000. However, GO was withdrawn from the US market in June 2010, because a large-scale clinical trial failed to show additive or synergistic effects with conventional chemotherapy for newly diagnosed AML. GO is currently available only in Japan. However, several large clinical studies have demonstrated beneficial effects of GO when added to chemotherapy for AML in recent years; therefore, reconsideration of GO availability is gaining attention. Therefore, the role and efficacy of GO as monotherapy or in combination therapy for de novo or relapsed AML should be positively investigated.