To assess the incidence of serious ventricular arrhythmias (SVA) during transient ischemic attacks (IA), 27 patients with severe angina at rest were submitted to Holter monitoring for a total period of 1344 hours. The recorded IA's were 565. To evaluate the time/relation between SVA and ST segment changes, the IA's complicated by SVA were divided in 2 parts: one for the upstroke and plateau phase of ST changes; Hse other for the period of resolution of ST changes and the first 3 minutes after ST return to baseline. SVA's were found in 12 of the 27 patients (44.4%), but only in 29 of the 565 IA's (5.1%). The recorded arrhythmias consisted of: frequent ventricular premature beats (VPB) (greater than or equal to 5/min) in 8 patients, multifocal VPB in 6, paired VPB in 10, ventricular tachycardia in 5, R on T phenomenon in 2. No correlations were found between the occurrence of SVA during IA and type of ST changes, location of ST changes, occurrence of the same arrhythmias outside the IA's. SVA's were significantly more frequent during IA associated with pain than during silent IA. When considering only IA associated with ST elevation, a relation was found between the occurrence of SVA and amplitude and duration of ST displacement. The same relation was not observed when IA's associated with ST depression were taken in consideration. Among the 29 IA's complicated by SVA, 22 showed the arrhythmias during the first phase (3.8% of all IA's) and 7 during the second phase (1.3% of all IA's); 6 of these 7 IA's were associated with ST elevation.