Introduction acute urinary tract obstruction (aUTO) is a common finding in older hospitalised patients. Anecdotal reports described hyponatraemia in patients with aUTO, which subsides rapidly with relief of the obstruction. The aim of this study was to determine the incidence, severity and subsequent correction of hyponatraemia in patients with aUTO. Methods this is a prospective, single-centre, observational study including inpatients in the internal medicine and geriatric wards. A total of 204 patients were investigated, 104 with aUTO and 100 controls. The prevalence, severity and associations of hyponatraemia between aUTO and control patients were compared. Results the incidence of hyponatraemia was similar in aUTO and control groups 28% versus 22%, respectively (P = 0.42). However, the incidence of severe hyponatraemia was significantly higher in the study group 7% versus 1% (P = 0.04). Mean sodium level was lower in hyponatremic patients with aUTO 127.7 ± 5.9 mEq/l versus 130.8 ± 3 mEq/l (P = 0.013). Serum sodium remained largely unchanged in the control group (∆Na 1.5 ± 2.3 mEq/l) but increased significantly within 48 hours following catheter insertion in patients with urinary retention (∆Na 5.3 ± 4.2 mEq/l overall in the aUTO group and 9.6 ± 3 mEq/l in those with urinary retention and severe hyponatraemia (P = 0.002)).