Disturbances in salt and water balances are relatively common in children after brain surgeries for suprasellar and pituitary tumours, presenting diagnostic and therapeutic challenges. Although hypernatraemia associated with central diabetes insipidus is commonly encountered, it is hyponatraemia (HN) that poses more of a diagnostic dilemma. The main differential diagnoses causing HN are the syndrome of inappropriate antidiuretic hormone secretion, marked by inappropriate retention of water, and cerebral salt wasting, characterized by polyuria and natriuresis. Diagnosis and management can be even more difficult when these conditions precede or coexist with each other. These diagnostic and therapeutic dilemmas are discussed in detail in this review.