Desmopressin, a synthetic antidiuretic hormone analogue, is the only drug currently approved for the treatment of nocturia associated with nocturnal polyuria or multiple sclerosis (MS). Compared with vasopressin, desmopressin has a longer lasting and more potent antidiuretic effect and is devoid of vasopressor and uterotonic effects. In two large, randomised, double-blind phase III trials in adults with nocturia associated with nocturnal polyuria, 3 weeks of oral desmopressin therapy was significantly more effective than placebo in reducing the mean number of nocturnal voids and in normalising the rate of nocturnal urine production. Beneficial effects of desmopressin on nocturia were maintained and increased in patients completing 10 or 12 months of further treatment in a nonblind extension of short-term trials. In randomised, double-blind trials in MS patients with nocturia, nasal desmopressin reduced the mean number of nocturnal voiding episodes by 31-54%. In both patient populations, desmopressin increased the initial sleep period or mean maximum period of uninterrupted sleep by approximately 2 hours, an outcome significantly greater than that achieved with placebo. In trials of < or =6 weeks duration in adults with nocturia, desmopressin was generally well tolerated. Most desmopressin-related adverse events were transient and mild or moderate in severity. Clinically significant hyponatraemia was reported in approximately 5% and required withdrawal from studies in < or =3% of patients.