It is well known that hematopoietic cytokine stimulation can cause diffuse increase of FDG accumulation in bone marrow on PET imaging, which simulates that seen in patients with bone marrow metastases. However, diffuse bone marrow FDG uptake can be caused by other etiologies. We report 2 patients who did not have a history of hematopoietic cytokine stimulation. The FDG PET images showed diffuse bone marrow FDG uptake, and the patients were diagnosed as having myelodysplastic syndromes. These cases demonstrate that diffuse FDG uptake by bone marrow can suggest neoplastic disease of the hematopoietic tissues.