Background: The t(1;19)(q23;p13), which can result in the TCF3-PBX1 chimeric gene, is one of the most frequent translocations in B-acute lymphoblastic leukemia (B-ALL) and is observed in both adult and pediatric populations at an overall frequency of 6%. It can occur in a balanced or unbalanced form and as a sole abnormality is associated with an intermediate prognosis. Additionally, this translocation is observed in the context of hyperdiploid B-ALL, in which case it is associated with a poor prognosis. However, due to different translocation partner genes at chromosomes 1 and 19, distinct subtypes of hyperdiploid B-ALL with t(1;19)/der(19)t(1;19) are recognized based on the presence or absence of the TCF3-PBX1 fusion gene, but the cytogenetic and etiologic differences between the two remain understudied.Findings: We report a case of an adult with a history of relapsed precursor B-ALL whose conventional cytogenetics showed an abnormal female karyotype with both hyperdiploidy and a t(1;19)(q23;p13). Fluorescence in situ hybridization (FISH) on previously G-banded metaphases using the LSI TCF3/PBX1 Dual Color, Dual Fusion Translocation Probe confirmed the presence of the TCF3-PBX1 gene fusion.Conclusions: This particular pattern with a TCF3-PBX1 fusion within the context of a hyperdiploid karyotype is seen in B-ALL and is usually associated with a poor outcome. This case is one of only a few cases with both hyperdiploidy and a confirmed TCF3-PBX1 fusion, demonstrating the importance of using FISH for proper molecular classification of these cases in order to distinguish them from those with hyperdiploidy but no TCF3-PBX1 fusion gene. Such molecular studies may provide insight into the precise differences between TCF3-PBX1 positive and negative hyperdiploid B-ALL bearing the t(1;19)(q23;p13).