A 85-year-old man presented with a 2-week history of malaise, confusion and agitation and 4 weeks of polydipsia and polyuria, with water intake of 6 l/day. His past medical history included hypertension, congestive cardiac failure, and chronic kidney disease. On examination he was hypervolaemic. Initial investigations were as follows: serum sodium 115 mmol/l, potassium 4.6 mmol/l, urea 8.1 mmol/l (2.57.8), creatinine 138 μmol/l (66112), serum osmolality 245 mOsmol/kg (275295), paired urine osmolality 169 mOsmol/kg (801200), and sodium 21 mmol/l. Fasting glucose, calcium, thyroid function, and 0900 h cortisol were all normal, excluding other causes of polydipsia and hyponatraemia. A presumptive diagnosis of primary polydipsia and dilutional hyponatraemia was made; the patient was unable to tolerate a formal water deprivation test. He was managed with fluid restriction. On this regime his serum sodium levels normalised but the patient remained incessantly thirsty and distressed. Liaison psychiatry assessment revealed a strong preoccupation with thirst, in the absence of obsessive thoughts, psychotic, mood, or anxiety disorder. He demonstrated severe cognitive impairment (blind Montreal Cognitive Assessment 4/22 and Frontal Assessment Battery 4/18). A collateral history suggested a long history of cognitive decline, and a shorter history of compulsion to drink consistently associated with worsening confusion. Mirtazapine was commenced to ease the compulsion and provide night sedation and we utilised sponges and ice cubes to alleviate thirst, with good effect. The patients sodium on discharge was 135 mmol/l. Our case is an unusual case of primary polydipsia in the absence of mental illness that was successfully managed with the above behavioural and pharmacological measures. Although primary polydipsia is common in patients with psychosis, there is little documentation of cases in the absence of psychopathology. Patients with dementia frequently present with reduced water intake but rarely with excess thirst.