42 patients aged over fifty years underwent continuous electrocardiographic monitoring before preparation for colonoscopy, after preparation and during and after colonoscopy. After preparation 11 patients, and during colonoscopy 12 patients, showed no electrocardiographic abnormalities. 31 patients developed or had increased electrocardiographic abnormalities after preparation and 30 patients during colonoscopy. There was no difference in the frequency of electrocardiographic abnormalities after preparation for colonoscopy and during colonoscopy. 68% of the 19 patients with previously diagnosed heart diseases had arrhythmias after preparation and 47% during colonoscopy, and they were thus significantly more frequent than in patients without heart diseases. Serious forms of arrhythmias also occurred more often in patients with heart diseases. None of the arrhythmias observed produced clinical symptoms or complications. Electrocardiographic monitoring during the whole length of preparation and during colonoscopy for all patients with heart diseases does not appear technically feasible. We therefore recommend it for patients with very severe heart diseases only.