BACKGROUND Diabetes insipidus (DI) is a syndrome characterised by polyuria which is almost always associated with polydipsia. The most frequent cause is central DI, which is the result of an inadequate secretion of the diuretic hormone, and diagnosis involves differentiating it from other causes of polyuria and polydipsia. METHODS The authors report the clinical case of a previously healthy 4 year old girl, who, in December 1998, was found to have intense polydipsia accompanied by polyuria. Behavioural treatment was begun as an answer to what was thought to be psychogenic polydipsia, although results were unsatisfactory and the patient was brought to the Paediatric Nephrology Service at Hospital Maria Pia in June 1999. CONCLUSIONS A clinical study, which included the water restriction test and concentration tests with desmopressin, enabled us to diagnose central DI. In spite of the results from a cranial NMR scan being normal, follow up time is still too short to classify the aetiology as idiopathic. The girl is asymptomatic under treatment with intranasal desmopressin. The favourable evolution in this case highlights the need to act in a thorough manner in the study of situations involving polyuria/polydipsia.