A woman in her late sixties presented with severe hyponatremia and acute kidney injury (AKI) as consequence of psychogenic polydipsia and acute urinary retention due to urinary tract infection. Urinary catheterization promptly drained 5.5 L of urine with resulting polyuria, leading to an initial swift raise in plasma (P) sodium concentration, disregarding the course of fluid resuscitation. After the polyuric phase, normal range P-sodium levels were reestablished by oral water restriction. Treatment with psychoactive drugs, e.g., zuclopentixol, may have contributed to the severity of the condition. There are few published reports regarding water intoxication and urinary retention, but none reflecting severe hyponatremia precipitated by acute urinary retention in a patient with polydipsia. By this report, we illustrate the detrimental consequences on water and electrolyte homeostasis of urinary retention and polydipsia resulting in acute water intoxication. The purpose of presenting this case is firstly to draw attention to the potentially fatal combination of polydipsia and postrenal acute kidney injury, where the kidneys are unable to correct the enormous excess water, then to focus on the difficulty in correcting hypervolemic hyponatraemia in the context of polyuria after relief of urinary retention, and finally, to point out that patients in treatment with antipsychotics may have further worsening of electrolyte derangement.