An infant with congenital renal salt-losing syndrome is reported. The importance of recognizing this rare disorder is emphasized because early detection, and treatment with sodium supplementation, results in marked growth improvement and may prevent volume depletion, electrolyte disturbance, and cardiovascular collapse. The clinical features and biochemical abnormalities seen in the congenital renal salt-losing syndrome are described. In this condition, natriuresis occurs in the face of normal renal and adrenal function and, as the child gets older, spontaneous resolution occurs. Other causes of hyponatraemia in infancy are discussed.