Hyponatraemia implies water retention in excess of sodium with or without increased loss of sodium from the body; extracellular fluid volume may be increased, normal or reduced. It has many causes which are briefly reviewed. Among these is the rare syndrome of inappropriate secretion of antidiuretic hormone (SIADH). It is suggested that SIADH is often diagnosed incorrectly because the raised ADH levels are appropriate for the volume status of the child. Precision in the diagnosis is important because whilst water restriction is necessary for the treatment of SIADH, other measures including the administration of extra fluid are often required if the raised ADH is appropriate. Hyponatraemia in the newborn may be caused by prerenal failure, renal failure or renal sodium wasting which is common in premature infants. Careful control of sodium intake as well as water intake is vital in this age group. Surgery is associated with water retention, but recent studies suggest that ADH levels are raised post-operatively because of volume depletion and that present recommendations for fluid therapy during and following surgery are inadequate. The use of electrolyte-free dextrose solutions should be abandoned and more liberal use of physiological saline or colloid is recommended.