Four apparently healthy young adults with vague chest symptoms during the day, two of whom had infrequent syncope while ambulatory at night, had periods of asystole up to nine seconds in duration occurring repeatedly during rapid-eye-movement (REM) sleep. Extensive evaluations, including electrophysiologic studies in two patients, were normal. It is therefore suggested that the underlying pathophysiology involved autonomic dysfunction. REM sleep-related sinus arrests such as these, which may occur in apparently healthy subjects but are undetected, may explain some cases of sudden, unexpected death during sleep. Polygraphic monitoring during sleep may be helpful in delineating the pathophysiology of the sleep-related arrhythmia in persons with daytime cardiac arrhythmias thought to be secondary to abnormal vagal tone. The possibility of nocturnal asystole should be considered in patients such as those described here.