Atrial fibrillation (AF) is intermittent in 30% of patients with cardioembolic stroke and, therefore, might not be seen in a single standard ECG recording. The aim of this study was to evaluate if prolonged ECG monitoring (96 h) finds episodes of intermittent AF beyond the 24 h ECG monitoring in patients with cryptogenic stroke or transient ischemic attack (TIA). We prospectively evaluated consecutive patients affected by cryptogenic stroke or TIA who had sinus rhythm on a 12-lead ECG on admission, and during ECG monitoring performed in the acute phase (for at least 24 h). Patients had continuous 96 h Holter ECG monitoring within 30 days from stroke onset. 114 patients were included in the study (mean age 63.1 ± 15.1, 59 males). AF was found in 29 patients (24.3%). In 20 patients, AF was found in the first 24 h of recording, and in nine patients after 24 h. In addition, several other dysrhythmias such as supraventricular ectopic activity (33), ventricular tachycardia (10), sinus pause (4) and sinus-atrial block (1) were found. In patients with cryptogenic stroke or TIA, 96 h ECG monitoring detected a high rate of AF. One-third of AF was seen beyond 24 h of ECG monitoring.