We investigated the incidence of painless (silent) myocardial ischemia, manifested as S-T segment deviation, by Holter ECG monitoring in patients with chronic renal failure undergoing regular hemodialysis. Forty-five patients underwent Holter ECG monitoring for a continuous 48-hour period covering dialysis and the intermediate period of everyday activity at home. ECG criteria for ischemia were found in 15.5% of patients mainly during and immediately after dialysis with a simultaneous increase of R,S,R + S amplitude. There was no correlation of S-T segment deviation with the existence of cardiac dysfunction and coronary artery disease proved by hemodynamic and angiographic studies. It is concluded that hemodialysis itself seems to play an important role in the genesis of the above ECG findings, possibly by means of serum K and Mg changes.