The development of Moyamoya syndrome (MMS) after cranial irradiation for pediatric tumors has been well established. However, information on the development of MMS after proton beam radiotherapy is sparse. We present the case of a 2-year-old child who developed radiation-induced MMS after treatment with proton beam therapy. Pediatr Blood Cancer 2014; 61:1490–1492. © 2014 Wiley Periodicals, Inc.