The translocation t(1;19)(q23;p13) is found in 3-5% of all acute lymphoblastic leukaemias (ALL) and results in the expression of an E2A/PBX1 hybrid gene transcript. This translocation is very closely associated with a pre-B phenotype. t(14;18) is associated with follicular B-cell lymphoma and is characterized by over-expression of the bcl-2 oncogene. We describe a case of ALL in an adult with a mature B-cell immunophenotype and a single abnormal cell line with a complex karyotype showing both t(1;19) and t(14;18). Two reports of this phenomenon have been published previously and molecular analysis, where performed, showed the E2A gene was not rearranged, suggesting the t(1;19) was a molecular variant of the established translocation. In contrast, molecular analysis of our case demonstrated expression of the E2A/PBX1 fusion transcript typically associated with t(1;19) in pre-B ALL but showed it to be present at an extremely low level, despite the abnormal karyotype being found in the majority of metaphase cells. Analysis of bcl-2 expression showed a significant up-regulation. A down-regulation of the E2A/PBX1 hybrid gene as a consequence of the enhanced expression of bcl-2 may be a possible mechanism for this finding.