It has been reported that Epworth Sleepiness Scale (ESS) values in untreated patients with obstructive sleep apnea–hypopnea syndrome (OSAHS) are significantly lower when measured prior to nasal continuous positive airway pressure (nCPAP) treatment than those recalled after initiation of treatment. This has been termed “response shift” in ESS. In this study, we investigated the response shift in ESS among a large number of patients with OSAHS, and performed additional analysis to identify any predictor for a significant response shift. We enrolled 232 consecutive moderate-to-severe OSAHS patients scheduled to commence nCPAP. All patients completed the ESS before initiation of nCPAP (Pre-ESS). At 1 month after initiation of nCPAP, patients completed the ESS once according to their current perceptions (Post-ESS), and again recalling sleepiness before initiation of nCPAP (Post Pre-ESS). We compared the Pre-ESS and Post Pre-ESS values in order to evaluate the response shift in ESS. The Post Pre-ESS value was significantly higher than mean Pre-ESS value (11.7 ± 5.6 vs 10.0 ± 4.7, respectively, P < 0.0001) and the response shift in ESS was observed. Multiple regression analysis between patient characteristics and the degree of the response shift showed that the body mass index (BMI) and arousal index (ArI) had independently significant positive correlation with the degree of the response shift. In conclusion, there was a response shift in ESS among OSAHS patients, particularly in those with higher BMI and ArI.