Leptin is known to regulate food intake and energy expenditure. Moreover, recent studies in rodents have shown that leptin in high doses decreases urine osmolality and increases water intake, together with renal sodium and water excretion. Since it is not clear whether leptin is involved in either the pathogenesis or in resulting metabolic abnormalities in diabetes insipidus, we measured leptin levels in 16 male patients with idiopathic central diabetes insipidus (DI) in comparison to a group of age- and BMI-matched male control subjects (no.=30). We found that plasma leptin concentration did not differ between patients with DI (4.45+/-3.24 microg/l) and healthy control group (3.69+/-2.81 microg/l; t test, p=0.41). Urine osmolality, plasma osmolality and urine volume of the patients with DI was not correlated to leptin. However, leptin was positively associated with body mass index (r=0.67; p=0.005). We conclude that leptin plasma concentrations are unchanged in diabetes insipidus. Furthermore, the lack of correlation between plasma leptin and urine osmolality, plasma osmolality or urine volume suggests that ADH deficiency does not have any impact on the plasma leptin levels.