Abstract. The value of a 7-h water deprivation test incorporating urinary osmolality and urinary arginine vasopressin (AVP) measurements was investigated in 20 children with suspected anterior or posterior pituitary dysfunction (group A) and 11 presenting with polyuria and polydipsia (group B). A control group of 16 healthy children was also studied. Urinary osmolalities in the control subjects after 7 h of water deprivation were 827–1136 mosmol/kg and urinary AVP 114–320 pmol/l. Of the group A patients, 5 had symptomatic diabetes insipidus with urinary osmolalities < 300 mosmol/kg, and urinary AVP concentrations of < 10 pmol/l, and 5 had normal urinary concentrating ability. The other 10 patients had varying degrees of partial diabetes insipidus (urinary AVP 6–53 pmol/l) although in 3 urinary concentrating ability was well maintained (osmolality 650–747 mosmol/kg). In group B, a diagnosis of compulsive water drinking was made in 9 patients, 1 had nephrogenic diabetes insipidus (urinary osmolality 68 mosmol/kg, AVP 782 pmol/l), and the final patient had transient diabetes insipidus. The test described was easy to perform and well tolerated even in young children. Using this test alone, it was possible to identify patients with partial defects of posterior pituitary function even when urinary concentrating ability was maintained, as well as those with complete cranial diabetes insipidus, nephrogenic diabetes insipidus, and compulsive water drinking.