RAS network activation is common in human cancers, and in acute myeloid leukemia (AML) this activation is achieved mainly through gain-of-function mutations in KRAS, NRAS or the receptor tyrosine kinase FLT3. We show that in mice, premalignant myeloid cells harboring a Kras(G12D) allele retained low levels of Ras signaling owing to negative feedback involving Spry4 that prevented transformation. In humans, SPRY4 is located on chromosome 5q, a region affected by large heterozygous deletions that are associated with aggressive disease in which gain-of-function mutations in the RAS pathway are rare. These 5q deletions often co-occur with chromosome 17 alterations involving the deletion of NF1 (another RAS negative regulator) and TP53. Accordingly, combined suppression of Spry4, Nf1 and p53 produces high levels of Ras signaling and drives AML in mice. Thus, SPRY4 is a tumor suppressor at 5q whose disruption contributes to a lethal AML subtype that appears to acquire RAS pathway activation through a loss of negative regulators.