Two cases of medulloblastoma are described which show positive immunostaining for glial fibrillary acidic protein (GFAP) in many cells. The surgical and autopsy specimens were examined by the indirect immunoperoxidase method. Positive staining for GFAP was demonstrated in the small, round to polygonal cells in both surgical specimens and in the small spindle cells in the autopsy specimen of one case. In the small, round to polygonal cells positive GFAP was shown as a perinuclear brown rim or intracytoplasmic brown droplet. In the small spindle cells, the cytoplasm and the polar processes were stained. Except for GFAP staining, these positive cells were morphologically indistinguishable from the accompanying unstained cells, indicating that GFAP was expressed in the medulloblastoma cells. Considering that GFAP is specific for astrocytes, these findings suggest the potential of astrocytic differentiation in the neoplastic cells of these medulloblastomas. The findings obtained in other 28 medulloblastomas examined in parallel are also discussed briefly.