Adolescent Down syndrome with leukemia is very rare and distinctive disease. The association of Down syndrome and leukemia has been documented for over 50 years. The age of onset for leukemia in this disease is bimadal, peaking first in the newborn period and again at 36 years. The association risk of these two conditions-Down syndrome and leukemia - is about 10- to 20-fold higher than in the general population. This increased risk extends into adulthood. Down syndrome associated with acute leukemia has usually poor prognosis, but some cases went to spontaneous remission in previously published papers. The proportion of acute leukemia type of these patients shows no difference compared to non-Down syndrome leukemia. A 20 year old male patient was admitted to our hospital due to frequent common cold history and recently developed periumbilical pain and poor oral intake. He was born second of 5 siblings from normal parents by normal spontaneous vaginal delivery. Conaenital appearance was mongoloid face and simian crease in both hands. The facial appearance was hyperteloric eyes and opened mouth and small face, flat nose at a glance. His CBC showed severe anemia and abnormal leukocyte counts with differential counts. So he was admitted to Hemato-Oncology department of internal medicine and then we carefully did physical examination and laboratory tests. His clinical features, peripheral blood smear and bone marrow aspiration-cytology and biposy revealed acute lymphocytic leukemia (ALL) Ll subtype. The chromosome study revealed 47, XY, +21 in karyotype. Thus, he was diagnosed as ALL with Down syndrome. Treatment was not given to this patient due to incorporation of patient because of retarded mentality. The patient was expired about 4 month's later. Thus, we report the first case of acquired ALL in adolescent Down syndrome in Korea,