Abstract The results of 24-hour continuous electrocardiographic monitoring of 23 patients with documented sleep apnea syndrome were reviewed to evaluate the prevalence of cardiac arrhythmias and conduction disturbances in this disorder. During sleep, marked sinus arrhythmia (more than 30 beats/min variation) was found in 18 patients. Extreme sinus bradycardia (heart rate less than 30 beats/min) and sinus pauses (more than 1.8 sec) were found in only two patients. First-degree and type I second-degree atrioventricular block were found in another patient. There was a decrease in grade of ventricular ectopy from wakefulness to sleep. These data suggest that the prevalence of serious arrhythmias and conduction disturbances during sleep in patients with the sleep apnea syndrome is much lower than previously reported.