The primary objective of this study is to assess the long-term safety of arsenic trioxide (ATO) in acute promyelocytic leukemia (APL) patients when used in combination with all trans retinoic acid (ATRA) in a real-world setting. The secondary objective is to describe the effectiveness and safety of different dosing regimens of ATO in APL patients. This will be a retrospective cohort study using data from existing multinational prospective APL registries conducted in European countries where the product will be marketed as first-line therapy. Participants will be newly diagnosed, low-to-intermediate risk APL patients aged ≥ 18 years receiving first line treatment with ATRA+ATO or ATRA+chemotherapy. Cases of high risk APL (WBC count >10x10^3/µl) and APL relapse will be excluded. Approximately 640 patients will be included over a period of 5 years. Patient follow-up will begin at treatment initiation and will end either: after 5 years, upon loss to follow-up, or death, whichever occurs first. Assuming an attrition rate of 4% every 6 months, we estimate that durations of follow-up will be: 85 patients for 5 years, 184 for 4 years, 300 for 3 years, 436 for 2 years and 590 for 1 year. Study variables include: demographics, body weight, adverse events of special interest: differentiation syndrome, creatinine (renal and urinary disorders), bilirubin (hepatobiliary disorders), aspartate amino transferase/alanine amino transferase ratio (hepatobiliary disorders), neurotoxicity, hemorrhage, sepsis (Infections and infestations), QTc prolongation and cardiac events including congestive heart failure, unexpected serious adverse events including grading and relationship, development of secondary malignancies, development of therapy-related myelodysplastic syndrome and acute myeloid leukemia, death, and cause of death. Incidence rates of primary and secondary endpoints will be assessed and summary statistics will be reported. Analyses will be performed by dosing schedule.