Proliferating cell nuclear antigen (PCNA) is a celb cycle-related nuclear protein, which is essential for DNA synthesis. It could reflect cell proliferating activity. And there're reports that it has got relationship with prognosis of many tumors. Immunohistochemistry was used to detect the expression of PCNA in medulloblastoma and to study its relationship with prognosis, differentiation, histological classification, size and mitotic index. The results showed that the expression of PCNA is related significantly only with the prognosis, i.e. the lower the expression, the better the prognosis. But no relationship of the expression of PCNA with differentiation, histological classification, size and mitotic index of the tumor could be found.