Sleep apnea syndrome (SAS) is common in patients with end-stage renal disease (ESRD). Although the treatment of choice is continuous positive airway pressure (CPAP) particularly for obstructive SAS, long-term compliance is not satisfactory. We investigated the effectiveness of nocturnal oxygen therapy on sleep apnea and autonomic nervous dysfunction in peritoneal dialysis (PD) patients with SAS. 40 patients on PD in our outpatient clinic were screened for SAS by pulse oximetry. We set the indication for nocturnal oxygen therapy at 4% oxygen desaturation index (4% ODI; defined as the number of falls of oxygen saturation > or = 4% per hour) > 5 or average nocturnal saturation < 95%. For SAS patients, 2 l per minute of oxygen was given during sleep and polysomnography was performed before and 1 month after oxygen administration. The heart rate variability was analyzed to assess autonomic nervous activity. 23 patients fulfilled the indication for oxygen therapy and 11 patients agreed to participate in the study. After oxygen therapy, the apnea-hypopnea index (AHI) and the frequencies of hypopnea and central apnea were significantly decreased (AHI: from 31.1 +/- 8.8 to 12.7 +/- 8.5, p < 0.01; hypopnea: from 19.5 +/- 4.3 to 3.5 +/- 3.2, p < 0.01; central apnea: from 4.0 +/- 4.0 to 0.8 +/- 1.2, p < 0.05), whereas that of obstructive apnea was not changed. An analysis of heart rate variability showed that oxygen therapy did not alter autonomic activity after 1 month of oxygen therapy. Nocturnal oxygen therapy decreases hypopnea and central apnea in PD patients with SAS. Nocturnal oxygen therapy may be useful for the treatment of SAS in PD patients, particularly when central apnea and hypopnea are predominant.