A 40-year-old Caucasian man developed excessive thirst and polyuria particularly at night over the preceding 6 months. He had been taking lithium for 16 years for the treatment of bipolar affective disorder. Investigations revealed subnormal maximum urinary concentrating ability after 8 hours of water deprivation and only a borderline response of urine osmolality to exogenous desmopressin given by intramuscular injection. A plasma copeptin concentration was elevated at 23 pmol/L. These results were consistent with partial nephrogenic diabetes insipidus. He was encouraged to increase his water intake as dictated by his thirst. In addition, he received amiloride with some improvement in his symptoms. Clinicians should be aware of the risk of nephrogenic diabetes insipidus with long-term lithium use and seek confirmation by a supervised water deprivation test augmented with a baseline plasma copeptin. If increased water intake is insufficient to control symptoms, amiloride may be considered.