This randomized phase 3 clinical trial evaluated the potential benefit of the addition of gemtuzumab ozogamicin (GO) to standard induction and postconsolidation therapy in patients with acute myeloid leukemia. Patients were randomly assigned to receive daunorubicin (45 mg/m2 per day on days 1, 2, and 3), cytarabine (100 mg/m2 per day by continuous infusion on days 1–7), and GO (6 mg/m2 on day 4; DA+GO) vs standard induction therapy with daunorubicin (60 mg/m2 per day on days 1, 2, and 3) and cytarabine alone (DA). Patients who achieved complete remission (CR) received 3 courses of high-dose cytarabine. Those remaining in CR after consolidation were randomly assigned to receive either no additional therapy or 3 doses of GO (5 mg/m2 every 28 days). From August 2004 until August 2009, 637 patients were registered for induction. The CR rate was 69% for DA+GO and 70% for DA (P = .59). Among those who achieved a CR, the 5-year relapse-free survival rate was 43% in the DA+GO group and 42% in the DA group (P = .40). The 5-year overall survival rate was 46% in the DA+GO group and 50% in the DA group (P = .85). One hundred seventy-four patients in CR after consolidation underwent the postconsolidation randomization. Disease-free survival was not improved with postconsolidation GO (HR, 1.48; P = .97). In this study, the addition of GO to induction or postconsolidation therapy failed to show improvement in CR rate, disease-free survival, or overall survival. This trial is registered with www.clinicaltrials.gov as #NCT00085709.