We describe a 21-year-old man presenting with proximal muscle weakness associated with hypernatremia. His manifestations other than muscle weakness included dry skin, loss of axillary and pubic hair, decreased libido and loss of thirst sensation. His serum sodium level was elevated to 169-171 mEq./l but all other electrolytes were normal. In addition, serum CK was elevated and an EMG study showed myogenic changes. Endocrinological studies revealed hypothalamic hypopituitarism, while MRI revealed a suprasellar mass. A partial correction of hypernatremia led to an immediate recovery of the muscle weakness as well as a normalization of both the serum CK level and EMG findings, suggesting a direct association between the muscle weakness and hypernatremia. The phosphocreatine/inorganic phosphorus (PCr/Pi) ratios in the resting calf muscle, obtained using 31P magnetic resonance spectroscopy (MRS), were very low during the state of muscle weakness, while they returned to nearly normal values after clinical improvement, suggesting that the muscle weakness in hypernatremic state was caused by a depletion of the intramuscular energy stores, probably due to an overworking Na-K pump to correct the intracellular electrolyte imbalance.