We reported a 17-year-old girl with relapsed acute promyelocytic leukemia (APL) who achieved complete remission and has been received maintenance therapy with all-trans retinoic acid (ATRA). The patient was diagnosed as APL in 1986. The ANLL 861 protocol of the Children's Cancer and Leukemia Study Group induced complete remission, and the chemotherapy was discontinued in 1989. However, she suffered a relapse with APL in 1991 and begun receiving ATRA (30 mg/m2/day) therapy because of disseminated intravascular coagulation. Bleeding tendency was discontinued by day 5. During the treatment, the white blood cell count increased markedly to 35,510 per microliters on 15th day, however she achieved complete remission morphologically on day 18. After informed consent was obtained from the family, she has been given ATRA orally for more than three years at the time of this report. The pharmacokinetics examination (ATRA 20 mg/m2 single per os) was performed 12 and 22 months after the induction therapy. The each peak plasma level of ATRA was 89 and 149 ng/ml. The concentration of ATRA has yet reached a level despite the continuous ATRA therapy. We considered that it may be useful to monitor plasma levels of ATRA during the treatment.