The investigation included 500 inpatients, in a Central Hospital. For each of the patients, two ECG registrations were made each time an electrocardiogram was requested and without any modifications between each registration. All electrocardiograms were examined for variations in cardiacrhythm and QRST morphology. In 19.8% of the patients, this double registration provided some new information. The cardiacrhythm was changed in 16.4%, and this was of clinical significance in 15.2% of these. The figure corresponded to 1.6% of all the ECG's. The QRST morphology had altered between the registrations in 5.4%. Among the alterations, 46.8% were of clinical significance, corresponding to the presence of clinically important information in 3% of all the patients investigated, if an ECG registration was repeated within one minute. In one out of every fifth ECG registration it would be possible to obtain some new information, if the registration was repeated within one minute.