Ten adolescent and preadolescent patients who underwent posterior spinal fusion for idiopathic scoliosis or spondylolisthesis were examined for the presence or absence of the postoperative syndrome of inappropriate antidiuretic-hormone secretion. For each patient, levels of serum sodium, serum osmolality, urine sodium, urine osmolality, and serum antidiuretic hormone were obtained preoperatively and postoperatively. The investigation clearly showed that the syndrome occurred in each patient, with the level of serum antidiuretic hormone being highest within a few hours postoperatively. This resulted in a reduction of urinary output, the reduction being maximum on the day of operation and the output gradually rising to normal over the next three days. This syndrome and its associated low postoperative urinary output is common after spinal fusion and should be treated with restriction of fluids rather than administration of increased amounts of fluid.