I read with interest the article by Kawata and colleagues1 about daytime hypercapnia in obstructive sleep apnea syndrome (OSAS). Based on the response to continuous positive airway pressure (CPAP) therapy, the authors classified the patients as good responders (a decrease in Paco2 by 5 mm Hg) or poor responders (a decrease of < 5 mm Hg). A similar apnea-hypopnea index (AHI) was reported in the two groups. However, average and lowest nocturnal arterial oxygen saturation (Sao2) values were significantly lower in the poor responders.,1The investigators did not rule out the possibility of coexisting severe obesity-hypoventilation syndrome (OHS) in the poor responders. It is well documented that 70 to 90% of OHS patients have predominantly obstructive events during sleep.2Additionally, OHS patients exhibit lower nocturnal and mean Sao2 compared to patients with pure OSAS matched for AHI.4 Therefore, it is possible that OHS was more prevalent or more severe among the poor responders, which in turn might have accounted for the higher daytime Paco2. A recent study5 suggests that CPAP may not be effective in severe cases of OHS. Additionally, the authors did not report desaturation index, nocturnal carbon dioxide, and AHI in the poor responders after applying CPAP. As patients with hypoventilation may have frequent episodes of desaturation without obstructive apneic or hypopneic events, it becomes important to compare the desaturation index in both groups. The fact that poor and good responders had similar AHI does not imply that they have the same underlying sleep-disordered breathing.3–4 The response to CPAP therapy in OHS patients has not been well investigated, and no study has demonstrated the intermediate and long-term effects of CPAP in these patients. Future studies aiming to investigate the physiologic responses to CPAP in patients with sleep-disordered breathing should differentiate between patients with pure OSAS and patients with OHS and OSA.