The significance of exercise-induced ST segment depression in supraventricular extrasystoles (STx), in the preceding sinus beats (STs), as well as the significance of the difference between the two of them (STx-s), was studied in 96 patients with angiographically documented coronary artery disease (CAD) (group A)--34 with myocardial infarction (group A1) and 62 without (group A2)--compared to 37 subjects with normal coronary arteries (group B). All patients had supraventricular extrasystoles during exercise testing, the results of which were positive in 72 (75%) patients in group A and six (16.2%) patients in group B (sensitivity 75%, specificity 84%). Among patients in group A STx was greater than STs (1.7 +/- 1.0 vs 1.2 +/- 0.8 mm; p less than 0.001), and STx-s was positive in 70 (sensitivity 73%), whereas in group A2 there were 44 patients with these values (sensitivity 71%). Among patients in group B no statistically significant difference was found between STx and STx (0.4 +/- 0.6 vs 0.6 +/- 0.7 mm; p = NS), whereas STx-s was positive in three (specificity 92%). Among the 24 patients in group A with false negative results of exercise tests, 15 (62.5%) had a positive STx-s, whereas of the 17 patients in group A2 with false negative results, 10 (58.8%) had a positive STx-s. Among the six patients in group B with false positive exercise test results, the STx-s was positive in two.(ABSTRACT TRUNCATED AT 250 WORDS)