Although hyponatremia has been known to occur in patients with severe spinal cord injury with highly incidence, its mechanism has not been understood well. We examined a 64-year-old patient with severe hyponatremia after spinal cord injury by sequential measuring of the factors affecting water-electrolyte balance, such as antidiuretic hormone, renin, angiotensin II, atrial natriuretic peptide, and brain natriuretic peptide. The patient showed severe hypotension due to dysfunction of the sympathetic nerve. The hyponatremia gradually resolved with the improvement of sympathetic nerve function. According to those results, the sympathetic nerve dysfunction was thought to correlate with the hyponatremia, and it was suggested that the unknown sympathetic regulation of water-electrolyte balance existed.