By all-trans retinoic acid in combination with chemotherapy cure rates of 70 to 80% can be reached. On average, about 10% of patients still die in the early phase of the treatment and about 20 to 30% relapse. Molecular monitoring of the minimal residual disease (MRD) by qualitative nested RT-PCR and quantitative REAL-time PCR of PML/RARa allows to follow the individual kinetics of MRD and to identify patients with an imminent hematological relapse.