Ventricular arrhythmias during transient myocardial ischemia were studied in 60 patients with spontaneous angina and greater than or equal to 1 ischemic attack with ST-segment depression during 24-hour ambulatory electrocardiography. The patients were divided into 2 groups: group 1, 10 patients (17%) who developed ventricular arrhythmias during 26 of 92 (28%) ischemic attacks; and group 2, 50 patients who did not show this phenomenon. Daily ischemic attacks, total ischemic time and the proportion of symptomatic ischemic attacks were significantly greater (p less than 0.01) in group 1 versus group 2. In group 1 patients, ischemic attacks were found to have twice the duration in the presence of arrhythmias than in their absence (20.4 +/- 11.9 vs 9.1 +/- 8.4 minutes, p less than 0.01); arrhythmias were more common during symptomatic than during silent ischemic attacks (39 vs 13%, p less than 0.02). Arrhythmias occurred at the onset or peak of ST-segment depression (ischemia phase) in 6 cases (60%), during the resolution of ST-segment depression (recovery phase) in 2 cases (20%) and during both phases of ischemic attacks in the remaining 2 (20%). When compared to recovery phase arrhythmias, ischemia phase arrhythmias were characterized by a later onset time (173 +/- 144 vs 58 +/- 54 seconds, p less than 0.01) and a longer duration (105 +/- 107 vs 41 +/- 22 seconds, p less than 0.01). During the ischemia phase, 16 of 353 ventricular premature complexes initiated ventricular tachycardia, while during the recovery phase only 1 of 161 ventricular premature complexes resulted in ventricular tachycardia (4.5 vs 0.6%, p less than 0.02). Thus, ventricular arrhythmias may accompany spontaneous ischemic ST-segment depression, when the latter is recurrent, prolonged and symptomatic; arrhythmias are characterized by a greater frequency, duration and malignancy during the ischemia phase than during the recovery phase of ischemic attacks.