A 83-year-old patient with inflammatory syndrome, unknown fever, and intense lumbar pain underwent diagnostic imaging for suspicion of spondylodiscitis. Fused F-FDG PET/MRI images showed heterogeneous hypermetabolism in the lumbar spine characterized by infiltrative lesions and adjacent necrotic tissue. Peripheral blood analysis revealed leukoerythroblastic changes, whereas bone marrow biopsy was highly hypercellular and revealed an acute myeloid leukemia with 70% infiltration of blasts of erythroid lineage. The intensive lumbar pain and elevated lactate dehydrogenase level are suggestive of bone marrow necrosis as seen in the PET/MRI. Our case emphasizes the usefulness of fused PET/MRI imaging for metabolic characterization of AML.