Transient ST segment changes are thought to be an indicator of intermittent myocardial ischemia in patients with chronic stable angina pectoris, and ambulatory ECG monitoring is a potentially useful means of quantifying these changes. In order to evaluate their repeatability, the automated analytical system developed in our department was carefully validated. Ambulatory ST segment monitoring was then performed on two occasions, 6 weeks apart, in 16 patients with established chronic stable angina pectoris of at least 1 year's duration. Both periods of monitoring were undertaken after administration of placebo for 2 weeks, and two bipolar ECG leads (CM5 and CC5) were monitored on both occasions. The total number (mean +/- SEM) of episodes of ST segment depression greater than 1 mm and of 3 minutes' duration or more in 24 hours was 4.9 +/- 1.0 and 5.1 +/- 1.0 on the first and second recordings (p = NS), and the total duration of ST segment depression in 24 hours was 83.6 +/- 27.7 minutes and 78.9 +/- 20.8 minutes, (p = NS) respectively. The maximal depth of ST segment depression observed during 24 hours was 3.5 +/- 0.3 mm and 3.4 +/- 0.2 mm, respectively, in lead CM5 (p = NS), with similar findings from lead CC5. These results demonstrate that ambulatory ST segment depression observed during uncontrolled normal daily activity in patients with established chronic stable angina pectoris is reproducible. This technique could provide a valuable method for assessment of antianginal drugs and patterns of ST segment changes during normal daily activities, provided that manual checks to ensure artifact exclusion are used to supplement the automated analysis.