A higher complete remission (CR) rate was observed in patients with acute myeloid leukemia (AML) who, on a prior randomized study of induction therapy, received gemtuzumab ozogamicin (GO) plus interleukin-11 (IL-11) rather than GO alone. An adaptive randomized phase III study of the addition of IL-11 to idarubicin and cytarabine (IA) induction in 100 patients >/=50 years of age with AML or high-risk myelodysplastic syndrome (MDS) was conducted. Median patient age was 67 years (range 50-82). Twenty-four of the 45 (53%) patients randomized to IA plus IL-11 achieved CR. Eight (33%) subsequently relapsed, 4 (17%) died in CR; median time to treatment failure (TTF) was 37 weeks. Twenty-nine of the 55 (53%) patients treated without IL-11 achieved CR. Eight (28%) subsequently relapsed, 2 (7%) died in CR; median TTF was 46 weeks. Median overall survivals were 21 and 59 weeks for the IA plus IL-11 and IA cohorts, respectively (p=0.271, log rank test; 0.435, Gehan-Breslow test). Ten episodes of the following grade 3 or 4 cardiopulmonary toxicities were observed in patients receiving IA plus IL-11, 12 such episodes in those receiving IA alone: atrial fibrillation, pleural effusions, myocardial infarction, bradycardia or hypotension. Two patients in each arm experienced grade 3 peripheral edema. There was no significant difference in incidence of any grade 3 or 4 adverse event, including thrombocytopenia, between treatment arms. There was no significant impact on CR rates, TTF, survival, or toxicity of adding an IL-11 regimen to IA induction in patients >/=50 years of age with AML.