Patients with obstructive sleep apnea syndrome (OSAS) sometimes have atrioventricular (AV) block during sleep. However, significant resolution of such AV block with treatment for OSAS has been reported. On the other hand, during rapid eye movement (REM) sleep, conduction disturbances not associated with the apnea event can be observed, particularly in young healthy subjects. We report the case of a 67-year-old man with severe OSAS and 2:1 AV block that occurred only during the phasic events of REM sleep; continuous positive airway pressure (CPAP) treatment did not result in resolution of the AV block. No specific abnormalities were found on cardiac evaluation. Based on the analysis of overnight heart rate variability, CPAP treatment resulted in a markedly reduced ratio of low-frequency to high-frequency power and an increased high-frequency power, though high-frequency power was not increased during REM sleep on CPAP.