Retinoic acid syndrome (RAS) is a potentially life-threatening complication observed in patients with acute promyelocytic leukemia (APML) and first thought to be specifically associated with all-trans retinoic acid (ATRA) (also known as tretinoin) treatment. Subsequently, so-called RAS was recognized in APML patients who had been treated with another highly efficacious drug, arsenic trioxide, and yet did not appear in patients treated with tretinoin for other disorders. These facts and others support the notion that RAS depends on the presence of the malignant promyelocytes. This has led to the growing deprecation of the term 'retinoic acid syndrome' and to an increasing use of the term differentiation syndrome to signify this APML treatment complication.