t(8;21)(q22;q22) giving rise to RUNX1/RUNX1T1 fusion transcript is a recurrent non-random chromosomal translocation, accounting for approximately 5% of cases of acute myeloid leukemia and 10% of acute myeloid leukemia with maturation. Studies have demonstrated so far that t(8;21)(q22;q22) occurs only in acute myeloid leukemia, and B lymphoblastic leukemia with t(8;21)(q22;q22) has not been reported in the literature. In the present study, we report a 44-year-old woman with a diagnosis of a B lymphoblastic leukemia based on morphology and immunophenotype. Conventional cytogenetic studies have shown a complex cytogenetic abnormality, notably and surprisingly, a t(8;21)(q22;q22) translocation. Interphase and metaphase fluorescent in situ hybridization have revealed a RUNX1/RUNX1T1 fusion signal on derivative chromosome 8 but not on chromosome 21, confirming the unbalanced translocation between chromosomes 8q22 and 21q22 involving both the RUNX1 and RUNX1T1 genes. The significance of this novel finding and its clinical applications has been further discussed. © 2010 Elsevier Inc. All rights reserved.