Chiasmatic gliomas with metastatic spread are rare in children and are usually associated with diencephalic syndrome. They are mostly pilocytic astrocytomas and their transformation to high-grade astrocytomas has never previously been reported in the pediatric population. We report leptomeningeal spread of a chiasmatic pilocytic astrocytoma in a child presenting with diencephalic syndrome. He was treated with chemotherapy and radiation. The tumor recurred with transformation into a high-grade astrocytoma. Radiation therapy may have played a role in transformation of the tumor, but more research is needed to further clarify the biological behavior of this tumor.