Individuals with constitutional trisomy 21 (Down syndrome) are at increased risk of developing acute leukaemias, both of myeloid and lymphoid lineage. Although the cause of leukaemia in Down syndrome (DS) remains unknown, potential candidate genes include the ones on chromosome 21, and in particular AML1, the rearrangement of which in the t(8,21) is associated with the French-American-British (FAB) classification M2 subtype of acute myeloid leukaemia (AML) in the general population and has been described in Down patients with AML-M2. Recently, a new rearrangement involving AML1, the t(12;21), producing the TEL/AML1 hybrid transcript, has been described by molecular analysis as the most recurrent genetic lesion in childhood acute lymphoblastic leukemia (ALL). In order to investigate whether the t(12;21) could give a molecular clue as to the precise basis of the etiologic association between DS and acute lymphoblastic leukemia, we tested a series of 11 consecutive cases of ALL in DS children for the presence of the TEL/AML1 transcript, by RT-PCR analysis. We report absence of the TEL/AML1 rearrangement among the 11 cases tested. This data may be suggestive of alternative pathways involved in the pathogenesis of ALL in children with constitutional trisomy 21.