Acute myeloid leukemia (AML) is a heterogeneous composition of biologically defined subgroups. Modern trials aim at developing subgroup-specific therapies. The German AML Cooperative Group 1999 trial asks three questions in a randomized factorial design: high-dose vs. standard-dose AraC during induction therapy; G-CSF priming vs. no G-CSF priming; and autologous stem cell transplantation vs. maintenance therapy. An interim analysis with 938 patients reveals subgroup-specific differences in treatment efficacies. Thus, the application of high-dose AraC during induction results in a superior outcome as compared to standard-dose AraC in patients with unfavorable prognosis but not in other patients. These results underline the need for large comprehensive trials to allow the detection of therapy effects in biologically defined subgroups of AML.