In the past, adverse cardiac effects of potassium depletion in non-digitalised patients and in patients without cardiac disease were considered to be minimal. However, several studies have now demonstrated a significant incidence of ventricular ectopic activity in diuretic-treated patients, particularly in those who become hypokalaemic. Potassium repletion with potassium-sparing diuretics or with potassium chloride supplementation has generally demonstrated a beneficial therapeutic effect in reducing ventricular ectopic activity. However, occasional patients may have persistent ventricular ectopic activity after diuretic therapy which may result from focal myocardial lesions associated with potassium depletion. In addition, factors other than potassium depletion appear to initiate or magnify diuretic-induced ventricular ectopic activity. Thus, to avoid these adverse cardiac effects, normokalaemia should be maintained during long term diuretic therapy.