Systemic mastocytosis (SM) associated with t(8;21) acute myeloid leukemia (AML) is very rare, and the D816 mutation of the KIT gene has previously been detected only in adult patients. We herein report the case of a 5-year-old female presenting with AML harboring t(8;21)(q22;q22). Her AML was refractory to chemotherapy, and bone marrow mastocytosis developed simultaneously at the initial diagnosis and during chemotherapy. The D816A mutation of KIT was detected. SM associated with t(8;21) AML, accompanied by a KIT mutation in children may result in a poor prognosis, despite the fact that t(8;21) AML are generally considered to have a favorable risk. Pediatr Blood Cancer 2012; 59: 1313–1316. © 2012 Wiley Periodicals, Inc.