We describe a complex case of hyponatraemia with two aetiologies. A 49-year-old man who drank 6 litres of dilute alcohol per day presented confused and oedematous with a serum sodium of 95 mmol/litre. Urine sodium was <10 mmol/litre and urine osmolality 440 mOsmol/kg. Chest x-ray demonstrated a globular heart. ECG showed saddle-shaped ST elevation. ECHO demonstrated a large pericardial effusion causing marked tamponade. Following pericardiocentesis there was a marked diuresis; serum sodium returned to normal after 2 weeks. A full recovery ensued. Cardiac tamponade is associated with antidiuresis via release of antidiuretic hormone (ADH). Tamponade is also associated with antinatriuresis. Antidiuresis and antinatriuresis usually balance in cardiac tamponade; excessive fluid intake may have caused an imbalance in this case.