To study the incidence and clinical significance of postshock ST segment elevations, we recorded 12-lead ECGs immediately after transthoracic direct-current electrical cardioversion in 146 patients with atrial fibrillation or flutter. Among 23 patients (19%), acute ST segment elevations amounted to 1.4 +/- 0.8 mV (mean +/- SD) and occurred after 243 +/- 114 joules (cumulative dose) in the precordial leads only in 14, concomitantly in precordial and inferior leads in two, and in the precordial and anterolateral leads in seven. They normalized within 1.5 +/- 0.8 minutes. Occurrence of ST segment elevation was related to previous pericardiotomy (p less than 0.00001) and age (p = 0.01). Estimation of myocardial enzyme release did not show evidence of myocardial damage. Patients with ST segment elevations had a lower conversion rate (48% versus 76%, p = 0.005). Long-term maintenance of sinus rhythm was also diminished (35% versus 59%, p = 0.03). Results of this study show that postcardioversion ST segment elevation occurs predominantly in postpericardiotomy patients and may be associated with an unfavorable arrhythmia prognosis.