The association between gastroesophageal reflux disease (GERD) and obstructive sleep apnea syndrome (OSAS) has recently been suggested. Patients with OSAS have a high frequency of nocturnal GER, and patients with GERD have various sleep-related symptom such as snoring and apnea. However, the causal relation between OSAS and GERD has not been sufficiently elucidated. The treatment of OSAS with nasal continuous positive airway pressure significantly reduces GERD symptom and nocturnal GER in both patients with and without OSAS. On the other hand, treatment of GERD in patients with OSAS improves the number of arousal during sleep, but one study showed a significant decline of apnea index. It is necessary to make further studies evaluating the association and treatment of OSAS and GERD.