Dilated cardiomyopathy may be primary or secondary. Although some causes are well known, such as toxic substances (alcohol, chemotherapy...) or viral infections, biochemical abnormalities are much less common. The authors report the case of a 58 year old woman with no previous history admitted to hospital for an inaugural episode of cardiac failure. The ECG showed sinus tachycardia with a long QT interval (560 mm) and a dilated hypokinetic cardiomyopathy with a left ventricular ejection fraction of 20%. The aetiological investigation showed severe hypocalcaemia (0.66 mmol/L) related to primary hypoparathyroidism. This is an important cause to remember because its treatment leads to correction of the cardiac disease, usually within weeks.