A previously healthy man in his 50s presented with headache and blurry vision. Laboratory data are summarized in the Table. He was diagnosed with acute myeloid leukemia (AML) complicated by leukostasis. Bone marrow karyotyping revealed a normal karyotype; 46 XX [20]. Multigene polymerase chain reaction (PCR) testing of the bone marrow identified a 44–base pair (bp) internal tandem duplication in the FLT3 gene (OMIM 136351), with an allelic ratio of 0.57 (range, 0.01 to >100). No other mutations on the panel (CEBPA, NPM1, KIT, IDH1, and IDH2) were detected.