Objective To study the incidence of leukocytosis and retinoic acid （RA） syndrome in newly diagnosed and relapsed acute promyelocytic leukemia （APL） patients treated with arsenic trioxide （ATO）. Methods Thirty patients with newly diagnosed or relapsed APL received ATO for remission induction at the dose of 10 mg/d. RA syndrome was defined when patient was with one or more of the following signs or symptoms: fever

dyspnea

serous cavity effusion

muscular pain

pulmonary infiltration

weight gain

or pulmonary infiltration on chest X-ray.Results Twenty-three （77%） patients achieved complete remission

mean time to remission was 37.1 days. Leukocytosis was observed in 14 （47%） patients

mean time to leukocytosis was 12.7 days

median baseline leukocyte count for patients with leukocytosis was 3.1×10 9/L

which was higher than that for patients who did not develop leukocytosis （2.6×10 9/L

z=-2.635

P=0.008）. No other cytotoxic therapy was administered

and the leukocytosis resolved in all cases. The RA syndrome was observed in 9 （30%） patients

mean time to diagnose of RA syndrome was 13.9 days

median baseline leukocyte count for patients with RA syndrome was 3.6×10 9/L

which was higher than that for patients who did not develop RA syndrome （2.6×10 9/L

z=-1.909

P=0.046）. No patient died of RA syndrome. Conclusion Leukocytosis and RA syndrome are associated with ATO and baseline leukocyte count respectively

and there is distinct link between leukocytosis and RA syndrome.