Patients with juvenile pilocytic astrocytoma (JPA) and neurofibromatosis type I (NF-1) tend to have a more indolent course than those with sporadic tumors. In rare circumstances, transformation to anaplastic pilocytic astrocytoma (APA) has been known to occur in sporadic cases and is associated with exposure to ionizing radiation. We present 2 patients with NF-1 who were initially diagnosed with JPA that later transformed to APA. Both patients were not exposed to ionizing radiation but instead received alkylator chemotherapy before transformation. Possibility of conversion to APA should be considered in patients with NF-1 and JPA who have rapid tumor recurrence.