The main features of Caceres-USPHS (version HP-3) program for automatic analysis of the ECG are reviewed. Our experience at the Ospedale Regionale di Udine has now reached a total of 40,000 tracings analyzed by such a program and with an HP 1530-A computer system. In a detailed study of 1,000 consecutive computer-analyzed tracings reviewed by three experienced cardiologists, 534 were normal and 466 abnormal. A high degree of specificity was detected in the normal group (only 1.5% of false negatives) while the sensivity was rather low. In the abnormal group there were 18.1% of false positives and while the agreement was 100% in complete bundle branch block and some types of infarction, it was 56% in atrial fibrillation and 47% in incomplete right bundle branch block. Program errors were more common in measurements and pattern recognition; they were unusual in the program logic (0.3%). A significant improvement over comparable data obtained one year previously was attributed to a better quality control in data recording and to increase physician acceptance of the program diagnostic criteria. On the current and past figures of accuracy, it is concluded that the version HP-3 of Caceres-USPHS program represents a step forward in computer-derived ECG interpretation. However more progress is still needed for a larger and more profitable clinical application of this ECG computer program.