Children with Down syndrome (DS) have a markedly increased risk of acute lymphoblastic and myeloid leukemias (ALL+AML). To identify chromosomal changes cooperating with +21 that may provide information on the pathogenesis of these leukemias, we analyzed 215 DS-ALL and 189 DS-AML. Unlike previous smaller series, a significant proportion of DS-ALL had the typical B-cell precursor ALL abnormalities high hyperdiploidy (HeH; 11%) and t(12;21)(p13;q22) (10%). The HeH DS-ALL were characterized by gains of the same chromosomes as non-DS-HeH, suggesting the same etiology/pathogenesis. In addition, specific genetic subtypes of DS-ALL were suggested by the significant overrepresentation of cases with +X, t(8;14)(q11;q32), and del(9p). Unlike DS-ALL, the common translocations associated with non-DS-AML were rare in DS-AML, which instead were characterized by the frequent presence of dup(1q), del(6q), del(7p), dup(7q), +8, +11, del(16q), and +21. This series of DS leukemias - the largest to date - reveals that DS-ALL is a heterogeneous disorder that comprises both t(12;21) and HeH as well as DS-related abnormalities. Furthermore, this analysis confirms that DS-AML is a distinct entity, originating through other genetic pathways than do non-DS-AML, and suggests that unbalanced changes such as dup(1q), +8, and +21 are involved in the leukemogenic process.