The present study aimed to identify optimal treatment intensity in children with mosaic Down syndrome (DS) and acute megakaryoblastic leukemia (AMKL). A retrospective review of AMKL patients was undertaken to identify mosaic DS children. Between November 1992 and November 2007, seven children were diagnosed as mosaic DS and AMKL. The median age at diagnosis was 29 months (range 4-34 months). Three patients had a past history of transient abnormal myelopoiesis. UPN1-4 were treated with intermediate-dose cytarabine and UPN4 received additional one course of high-dose cytarabine. All of these patients were remained in first CR. UPN5-7 were treated with high-dose cytarabine according to the AML99 protocol. UPN5 with GATA1 mutation suffered from acute pneumonia and pancreatitis and discontinued chemotherapy. UPN7 relapsed after cessation of chemotherapy and was rescued with allo-PBSCT. The cumulative doses of cytarabine were 3.5-10.65 g/m(2) in the UPN1-4 and 40.4-78.4 g/m(2) in the UPN5-7. The 8-year overall survival was 100% and the 8-year event-free survival 85.7%, respectively. Our retrospective study reveals that patients with mosaic DS and AMKL have a good prognosis. Reduction in intensity may work in patients with mosaic DS as well as with AML-DS.