Electrocardiographic ambulatory monitoring with a real-time analysis system. The analysis of records collected during long-term ambulatory electrocardiographic monitoring has traditionally involved the review of massive data, either manually or with the aid of interactive scanning computers. Many factors complicate this analysis, including the use of analog tape for storage of electrocardiographic waveforms, the need to analyze 100,000 waveforms from an average 24-hour study, and the need to deal with an interface that compresses 24 hours of data into as little as 6 minutes on a screen. Today, the computer incorporated in the monitor can scrutinize each cardiac cycle in real time. The system produces a statistical report based on every heart beat and also performs data reduction and storage of electrocardiograph samples. To assess real-time analysis we examined data collected from the Circadian CircaMed ambulatory electrocardiography system. We found that it could detect and quantify simple or complex ventricular ectopic beats, brady- or tachyarrhythmic events, and ST-segment deviation. One hundred fifty patients 21 to 85 years old with symptoms or clinical finding suggestive of ischemia, cardiac arrhythmia, or conductive defects were referred to our electrocardiography laboratory for ambulatory monitoring. The results demonstrate that this system can detect the full range of cardiac disease found with the traditional method. Of the 150 patients, ambulatory electrocardiographic tests were positive in 93 (62%). In addition, we developed a methodology for lead placement when using two bipolar leads, as is typical for ambulatory electrocardiography. We present a procedure for determining the optimum lead placement that is based on the patient's history and a 12-lead electrocardiogram. 