A study has been made of the accuracy of a computerized system of arrhythmia monitoring in the coronary-care unit of the Palo Alto Veterans Administration Hospital. The system provides beat-to-beat analysis of cardiac rhythm and QRS morphology and stores information for display of incidence of abnormalities and trend curves. The variables monitored by the system include: (1) heart rate, (2) early beats, (3) late beats, (4) abnormal QRS waveforms, and combinations of 2, 3, and 4. Accuracy analysis was performed using a standard ECG trace with a simultaneous record of computer classification of each QRS complex on a dual-channel recorder. Thirty-four patients with various cardiac arrhythmias were studied and 188,000 QRS complexes were evaluated. The number of true abnormal beats (abnormal in timing or waveform) recorded by the ECG was 12,194 out of 188,000 or 6.5 per cent. Computer classification accuracy was as follows: false positives (computer diagnosis abnormal but ECG diagnosis normal), 0.10 per cent (191 complexes) of all complexes and 1.56 per cent of true positives. False negatives (computer diagnosis normal but ECG diagnosis abnormal), 0.70 per cent (84 complexes) of abnormal beats. Computer monitoring and analysis of seven variables in cardiac rhythm and QRS waveform abnormalities can be performed with a high degree of accuracy using a compact computer system suitable for intensive-care units.