Purpose Expansion sphincter pharyngoplasty (ESP) is a common surgery for patients with obstructive sleep apnea (OSA) which aims to correct the obstruction at the palatal level. The effectiveness of ESP has been widely shown in the literature using surgical success rates, but to our knowledge, there is no research which documents the changes in the upper airway anatomy objectively. We aimed to demonstrate the effectiveness of expansion sphincter pharyngoplasty using acoustic pharyngometry. We also aimed to study the possible utility of acoustic pharyngometry in predicting surgical outcomes. Methods Pre- and post-operative acoustic pharyngometry and polysomnography data of patients who underwent expansion sphincter pharyngoplasty were compared prospectively. Minimum cross-sectional area (MCA) and total volume of the pharynx (TPV), apnea-hypopnea index (AHI), and surgical success rates were evaluated. Results Fifty-two patients with OSA were invited to this study, and 35 patients who agreed to participate were enrolled. All patients underwent ESP surgery. Surgical success rate was 63% according to Sher's criteria. The mean AHI of the patients decreased from 29.6 +/- 16.3 to 18.3 +/- 18.1. MCA increased from 1.1 +/- 0.4 to 2.3 +/- 0.4 cm(2), and TPV increased from 21.1 +/- 6.9 to 31.7 +/- 5.5 cm(3). Comparative analysis of the successful and unsuccessful groups yielded no significant differences between the groups concerning pre- and post-operative MCA and TPV or in mean changes in MCA and TPV achieved with the surgery. Conclusion Improvement in the upper airway anatomy by expansion sphincter pharyngoplasty can be clearly demonstrated using acoustic pharyngometry. Acoustic pharyngometry findings are quite similar in patients with successful and unsuccessful outcomes; therefore, pharyngometry findings cannot be used to predict surgical success; and surgical success cannot be solely attributed to the changes in MCA and TPV.