A 4-year-old male was admitted to the pediatric medicine service after a 5-day history of severe holocephalic headaches and an episode of emesis the day prior to admission. The headaches were progressive in severity, occurring in the early morning and often walking him up from sleep. No seizure activity, focal deficits, or infectious symptoms were reported by parents. His past medical history was significant for diagnosis of acute monocytic leukemia (AML) 1 year previously. Bone marrow karyotyping and interphase fluorescence in-situ hybridization at the time of initial presentation revealed AML with 11q23 abnormalities at the MLL (mixed lineage leukemia) gene, with t(1:17) and ins(10;11). He received four cycles of chemotherapy on the Children's Oncology Group (COG) protocol AAML1031, 1 with a line infection complicating his third cycle. After his fourth cycle, he was discharged home. Throughout his initial diagnosis and up to the time of current presentation-6 months after his last chemotherapy cyclehe did not demonstrate any signs or symptoms of neurologic abnormality and was central nervous system (CNS)-negative. His vitals were within normal limits, and a systemic physical examination was unremarkable. On neurologic exam, there were no noted cranial-nerve deficits or papilledema. No objective deficits in power, sensation, and cerebellar testing were elicited, and no upper motor neuron signs were elicited. Initial bloodwork showed no abnormalities.