Eight patients with monosymptomatic nocturnal enuresis (age 11-24 years) were investigated prior to and after 24 weeks of desmopressin treatment in order to evaluate the impact on the endogenous vasopressin secretion and urinary output. No effect on plasma vasopressin, diurnal urinary volume, and urinary osmolality were found after this long-term treatment. Overall no changes in either body weight, blood pressure, or hematological variables were demonstrated. This supports previous findings that the treatment appears to be well tolerated and free of side effects in longer term.