Acute leukemia with t(4;12)(q11-13;p12-13) is rare but has unique characteristics. The incidence of t(4;12) in acute leukemias was about 0.6% in our laboratory. Twelve patients with acute leukemia with t(4;12) have been reported until now. They included eight acute myeloid (AML: M0 2, M1 3, M2 1, M4 1, and M7 1), three acute lymphoblastic (ALL: L1) and one acute unclassified leukemia (AUL). There were some differences between adults and children with t(4;12). The eight adult patients included seven with AML and one with AUL, two of whom had a history of exposure to mutagenic agents and/or genotoxic therapy. Three patients had the CD7+ HLA-DR+ CD13+ CD34+ c-kit+ phenotype, suggesting that the leukemic cells were of stem cell origin. Four children expressed the B lymphoid phenotype (HLA-DR+ CD10+ CD19+) although one had myeloperoxidase positivity. It was difficult for adult patients to achieve complete remission with the usual therapy regimen, whereas children with t(4;12) seemed to be easier to treat. Rearrangement of the TEL gene located on the short arm of chromosome 12 (12p13), was investigated in two adult patients. FISH analysis using the YAC probe that covers the TEL gene region, revealed split signals in these patients, suggesting a break inside or near the TEL gene. The t(4;12) abnormality is associated with unique characteristics of acute leukemia namely stem cell or secondary AML in adults, and B lymphoid leukemia in children.