Cerebral salt wasting syndrome (CSWS) is defined as a renal loss of sodium in cerebral disorders causing hyponatremia and loss of extracellular fluid volume. Similar laboratory findings may be seen in other conditions such as syndrome of inappropriate antidiuretic hormone secretion (SIADH). A 58-year-old male visited our emergency department because of the sudden development of seizures during sleep. Magnetic resonance imaging revealed subtle high signal intensity in the right hippocampus on diffusion-weighted imaging. Ictal rhythmic discharges were observed in the concordant area. Altered metal status, polyuria and laboratory test findings including hyponatremia were compatible with CSWS. After hydration and salt replacement, his mental state and hyponatremia gradually recovered. For diagnosing CSWS, meticulous physical examinations including analysis of fluid balance are essential. CSWS should be considered in patients with hyponatremia and polyuria. Accurate diagnosis of CSWS and SIADH is crucial as the treatment plans for these two conditions are completely different.