Thirteen Sika deer (Cervus nippon Temminck) showing ataxia on a stock farm in the north eastern district of China were examined histopathologically. The principal pathological changes were spongy vacuolation and myelin deficiency in the white matter of the spinal cord and brain stem, fibrosis and rupture of the elastic lamina of the spinal arterioles, and mesothelial hyperplasia in the spinal arachnoid. Other findings included defective formation of the elastic laminae of the aorta, and the blood vessels in the kidney and lung, hemosiderosis in the spleen and liver, and lymphocyte depletion in the systemic lymph nodes. Copper concentrations were low in the serum and liver. In the white matter of the spinal cord and brain stem, demyelination appeared to coexist with dysmyelination and secondary myelin breakdown. It was inferred that decreased activity of copper containing enzyme induced various lesions. The possible role of copper deficiency in the pathogenesis of the ataxic conditions is discussed.