Objective: Under normal conditions, the kidneys handle a large variability of daily Na and volume intake without notable changes of blood pressure (BP). To increase the awareness of rare etiological factors during the workup of hypertension, we review the case of a patient with abnormal volume handling and reversible circadian hypertension due to chronic polydipsia. Design and method: A critical case analysis with discussion of the literature was performed. Results: A 76 year old female patient was evaluated for resistant hypertension. BP recordings showed normotension in the morning and a periodic daily BP rise with hypertension in the afternoon (usually >160 mmHg systolic) associated with chronic daytime polydipsia. The abnormal circadian BP oscillations disappeared when daily drinking volume was reduced to normal. She began voluntary polydipsia 12 years ago after an episode of painful uroltihiasis and advise to keep her daily drinking volume high for prevention. Four years earlier she had had an ischemic stroke in the vertebrobasilar territory. She received low dose perindopril and hydrochlorothiazide but the abnormal BP rise persisted also under amlodipine. Her clinical workup revealed a residual neurologic syndrome after stroke, mild renal insufficiency, and hypertensive heart disease with normal systolic function. Kidney ultrasound excluded renal artery stenosis. She showed normal serum eletrolytes and a normal plasma aldosterone/renin ratio when untreated. During extensive laboratory investigations, no other etiologic factor could be detected. The rapid BP rise suggested sympathetic nervous system activation. Polydipsia is frequently associated with electrolyte distrubances but pathologic BP oscillations are exceptional. Based on a review of the literature, we discuss abnormal renal volume control and an impaired central BP regulation and baroreflex function after brain stem infarction to explain the association of polydipsia with recurrent diurnal hypertension. Conclusions: Chronic polydipsia may be rare. However, it may occasionally represent a modifiable cause of BP elevations and treatment resistance in elderly patients particularly when volume handling is impaired by comorbidity.