Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder caused by mutations in the NF1 gene. Patients with NF1 have a higher risk to develop juvenile myelomonocytic leukemia (JMML) with a possible progression toward acute myeloid leukemia (AML). In an oligo array comparative genomic hybridization–based screening of 103 patients with pediatric T-cell acute lymphoblastic leukemia (T-ALL) and 71 patients with MLL-rearranged AML, a recurrent cryptic deletion, del(17)(q11.2), was identified in 3 patients with T-ALL and 2 patients with MLL-rearranged AML. This deletion has previously been described as a microdeletion of the NF1 region in patients with NF1. However, our patients lacked clinical NF1 symptoms. Mutation analysis in 4 of these del(17)(q11.2)-positive patients revealed that mutations in the remaining NF1 allele were present in 3 patients, confirming its role as a tumor-suppressor gene in cancer. In addition, NF1 inactivation was confirmed at the RNA expression level in 3 patients tested. Since the NF1 protein is a negative regulator of the RAS pathway (RAS-GTPase activating protein), homozygous NF1 inactivation represent a novel type I mutation in pediatric MLL-rearranged AML and T-ALL with a predicted frequency that is less than 10%. NF1 inactivation may provide an additional proliferative signal toward the development of leukemia.