Although new approaches have dramatically improved, the treatment of acute promyelocytic leukemia (APL) involvement of the central nervous system (CNS) confers a bad prognosis in the disease. Here, we report a patient who was diagnosed with relapsed APL preferentially involving the CNS. Treatment with arsenic trioxide led to impressive morphological changes in CNS cellularity consistent with the induction of a differentiation syndrome. Since arsenic trioxide could be identified in the CNS, we provide evidence that the drug can cross the blood-brain barrier and can be used for treatment of extramedullary APL.