This report presents the first study of cardiac arrhythmias and conduction disturbance in a large group (400 patients) with sleep apnea syndrome studied between 1974 and 1979 with 24-hour Holter electrocardiography and a simultaneously recorded polygraph during late afternoon or nocturnal sleep. Of the 400, 193 patients (48%) had cardiac arrhythmias during the recorded night. The mean number of apneic events, age, weight and lowest oxygen saturation during sleep were not significantly different in those with arrhythmias. The most significant abnormalities were unsustained ventricular tachycardia in 8 patients, sinus arrest that lasted for 2.5 to 13 seconds in 43 patients, and second-degree atrioventricular conduction block in 31. Seventy-five had frequent (greater than 2 beats/min) premature ventricular contractions during sleep. Fifty patients with significant arrhythmias had a tracheostomy and were monitored again after surgery. No arrhythmia was present in these patients except for premature ventricular contractions.