A comparative study was made between rhythm diagnosis by computer using the IBM ECG program (Bonner-I), and diagnosis by a physician, 2434 electrocardiograms (ECGs) recorded in Kitasato University Hospital from July to August 1978 were used. Of the 2434 cases the physician made a diagnosis of sinus rhythm in 2185 and of abnormal (other than sinus) rhythm in 249. Among theformer, 32 cases were erroneously classified into abnormal groups by the computer (false positive), and in 30 out of the latter 249 cases, the computer failed to detect abnormalities (false negative). In the remaining 219 cases, 33 of 57 cases of sinus rhythm with VPCs, 19 of 41 sinus rhythm with SVPCs, and 62 of 64 atrial fibrillation were correctly diagnosed by the computer. In 27 out of the 219 cases "undetermined rhythm" was printed out after an unsuccessful dominant rhythm determination by the computer. Cases of artificial pacemaker were not recognized by the computer, but were distributed into 7 classification categories. We were of the opinion that if the computer program had utilized the information on the duration of QRS complexes these cases could have been correctly classified. We repeated ECG sampling and analysis in the cases of complicated arrhythmias, because more valuable information was often supplied by the computer after successive ECG recording. From these results we concluded that the usefulness of the IBM ecg program was confirmed concerning the recognition of sinus rhythm, whereas the detection and identification of complex rhythm disturbances by the computer program was still in need of improvement.