We examined the incidence, clinical course, and outcome of patients with newly diagnosed acute promyelocytic leukemia (APL) who developed the retinoic acid syndrome (RAS) treated on the Intergroup Protocol 0129, which prospectively evaluated the role of alltrans retinoic acid (ATRA) alone during induction and as maintenance therapy. Forty-four of 167 (26%) patients receiving ATRA for induction developed the syndrome at a median of 11 days of ATRA (range, 2-47). The median white blood cell (WBC) count was 1,450/microL at diagnosis and was 31,000/microL (range, 6,800-72,000/microL) at the time the syndrome developed. ATRA was discontinued in 36 of the 44 patients (82%) and continued in 8 patients (18%), with subsequent resolution of the syndrome in 7 of the 8. ATRA was resumed in 19 of the 36 patients (53%) in whom ATRA was stopped and not in 17 (47%). The syndrome recurred in 3 of those 19 patients, with 1 death attributable to resumption of the drug. Ten of these 36 patients received chemotherapy without further ATRA, and 8 achieved complete remission (CR). Among 7 patients in whom ATRA was not restarted and were not treated with chemotherapy, 5 achieved CR and 2 died. Two deaths were definitely attributable to the syndrome. No patient receiving ATRA as maintenance developed the syndrome. (Blood. 2000;95:90-95)