The proliferative potential of a spinal nephroblastoma was studied in a young dog. A 4-month-old, female golden retriever showed developing deterioration in her gait and subsequent paralysis of her hind legs. At necropsy, a well-demarcated grayish brown tumor mass was found in the lumbar spinal cord segments between L2 and L3. Histologically, a blastemal cell tumor with a tubule- or glomeruli-like structure was found to be infiltrating intradurally. Proliferating cells at the S-phase, assessed using the bromodeoxyuridine (BrdU) labeling method, were seen occasionally in the tubular cells and glomeruli-like structures and were frequently seen in the blastemal cells. Immunohistochemically, the tubular epithelial cells were positive for cytokeratin, and the blastemal cells were positive for vimentin. The present tumor showed a high potential for growth and invasion, which suggests that it the potential to expand into the adjacent spinal cord.