Investigation of coronary heart disease manifesting as sudden death has highlighted the role of electrolyte disturbances in arrhythmogenesis. The identification of the 3 major cardiac risk factors--hyperlipidaemia, hypertension and smoking--does not fully explain sudden death in asymptomatic patients with an abnormal ECG. Sudden death is usually ascribed to cardiac arrhythmia whose pathogenesis has 3 possible mechanisms affecting the electrical properties of the heart. Thiazide diuretics are known to deplete potassium and magnesium in the body and while magnesium deficiency has been especially associated with cardiac rhythmicity, potassium levels modulate the cellular effects of calcium in the myocardium. In patients with ischaemic heart disease, both hypokalaemia and hypomagnesaemia correlate with the frequency of serious arrhythmias and even in ambulatory hypertensive patients on diuretics, it is important to preserve electrolyte homeostasis. There is, however evidence to suggest that some patients are more susceptible to diuretic-induced arrhythmias and in these patients even mild hypokalaemia can cause ventricular arrhythmias, and age may be a contributory factor. The risk of thiazide-induced arrhythmias has yet to be confirmed.