Hyponatremia can result from a wide range of causes. While hyponatremia is known to occur in patients with hypopituitarism, severe hyponatremia occurring as the presenting feature of hypopituitarism is very rare. We present two cases in which severe hyponatremia developed with weakness, light-headedness and seizure. The hyponatremia in these 2 cases mimicked the laboratory diagnostic criteria of a syndrome of inappropriate secretion of antidiuretic hormone (SIADH). However, the hormone studies displayed hypopituitarism. Hyponatremia was completely corrected after administering a supplement of prednisolone and L-thyroxine. Computerized tomography of the brain revealed an adenoma of the pituitary gland. These two cases illustrate that severe hyponatremia may be the presenting feature of clinically non-functional pituitary adenoma with hypopituitarism, which should be kept in mind in the differential diagnosis of hyponatremia mimicking SIADH.