A diagnosis of fat emboli can be suspected in a patient presenting with the typical symptoms of the fat embolism syndrome, but is rarely proved pathologically, except at autopsy. We described a 25-yr-old man with sickle cell anemia who developed an infarctive crisis complicated by unexplained fever, neurologic change, and respiratory abnormalities. Blood drawn from the femoral vein and examined cytopathologically yielded necrotic bone marrow elements admixed with fat. The cytologic finding of fat emboli from necrotic bone marrow provided the diagnosis and helped guide subsequent medical intervention. This sample test is recommended for patients at risk for fat emboli to aid in the clinical diagnosis.