A case of Prader-Willi syndrome who later developed hepatoblastoma is reported. Prader-Willi syndrome was suspected because of hypotonia, hypopigmentation, and undescended testes when he was a newborn infant. The diagnosis was confirmed by chromosome analysis, which showed 46XY del(15)(q11, q13). When he was 1 year 4 months old, a liver tumor and high serum AFP were found. At operation a large tumor arising from the caudate lobe was found and the tumor was totally resected. After completion of the hepatectomy, he developed circulatory collapse of unknown cause and died shortly after the operation. Histopathologic examination revealed that the tumor was composed of two components, well differentiated cells and poorly differentiated cells. The well differentiated part did not dominate the poorly differentiated part, so it was diagnosed as poorly differentiated hepatoblastoma. This is the first reported case of Prader-Willi syndrome with a pediatric malignant tumor.