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Abstract 762 Pediatric AML is a heterogeneous disease. We showed that differential outcomes of 11q23/MLL-rearranged AML depend on the fusion partner involved in the translocation in a large international retrospective study from 11 collaborative groups comprising 756 children (Balgobind et al, Blood 2009). In the current analysis we focused on the clinical characteristics and prognostic impact of additional cytogenetic aberrations (ACA). All patients with complete karyotypes (n=733) were centrally reviewed and classified for type and number of aberrations. Cases with 2 or more aberrations (including 11q23/MLL-rearrangement), were included in the ACA group (n=344/733, 47%). Numerical and structural ACA were divided into gains and losses of full or partial chromosomes, and balanced translocations were defined by the breakpoints. A complex karyotype was defined as 3 or more aberrations. ACA occurring in at least 10 patients were considered for statistical analysis, including differences in presenting characteristics (chi-square or Fisher's exact tests) and survival estimates (Kaplan-Meier method and Cox proportional hazards model for Event free survival (EFS), 5 year estimates and 95% confidence intervals (CI) are given). Multivariate analysis included the fusion partners with independent prognostic significance, as identified by Balgobind et al. Patients with ACA were found to be older (median age, 3.0 y vs 1.8 y, p=.001) and had a higher frequency of FAB M7 phenotype (4.5 % vs 1.1 %, p=.007) than those without ACA. For the complete cohort (n=733) estimates of EFS and overall survival (OS) were .44 and .56, and cumulative incidence of relapse (CIR) was .35 (standard errors .03). Patients with ACA showed significantly poorer clinical outcome than cases without ACA (EFS .38 vs .48, p=.002; OS .47 vs .62, p Disclosures: Smith: Pfizer, Inc:.