Patients with ALL rarely present with t(12;17)(p13;q21) as the primary clonal abnormality; this abnormality is associated with the expression of myeloid antigens. In this study, we have reported presumably the first case of this chromosomal abnormality in Korea, thereby facilitating the delineation of a distinct subtype of ALL. A 57-yr-old woman was referred to our hospital because of pancytopenia. Peripheral blood examination showed 55% blasts. The bone marrow was markedly hypercellular, and about 82.4% of all nucleated cells were blasts. The results of immunophenotyping and cytochemical staining suggested early precursor B-ALL. Cytogenetic analysis of the bone marrow cells showed a complex karyotype, including a reciprocal translocation between the short arm of chromosome 12 and the long arm of chromosome 17, t(12;17)(p13;q21). Data from array comparative genomic hybridization were almost consistent with the cytogenetic findings.