Recent studies have shown that lateral pharyngoplasty is an effective procedure to correct obstructive sleep apnea. The objective of the paper was to evaluate factors contributing to obstructive sleep apnea syndrome and identify the different severity categories of obstructive sleep apnea that could benefit from lateral pharyngoplasty. We included 60 patients undergoing uvulopalatopharyngoplasty with or without lateral pharyngoplasty from December 1, 2008 to May 31, 2012. Six who did not complete the post-operative survey were excluded. 54 participants were assigned alternatively to either uvulopalatopharyngoplasty alone (control group, n = 29) or to uvulopalatopharyngoplasty with lateral pharyngoplasty (intervention group, n = 25). The reduction of apnea-hypoxia index in the intervention group (49.3%) was higher than it was in the control group (30.4%), but the average value was not statistically significant (p = 0.088). Only patients with moderate obstructive sleep apnea in the intervention group achieved a significant reduction of apnea-hypoxia index (30.7 vs. 10.8%; p = 0.020). Factors such as body mass index, nadir/mean SpO2, snoring index, Epworth sleepiness scale and periodic limb movement index did not change significantly following lateral pharyngoplasty. To evaluate patients' upper airway anatomy, we employed cephalometry, and found that the retroglossal space played a critical role in severe obstructive sleep apnea. Our study showed that only patients in the moderate category can be benefited from adding lateral pharyngoplasty to uvulopalatopharyngoplasty; however, patients in the mild or severe category did not benefit from adding this procedure. This information is vital when considering surgery for patients with obstructive sleep apnea syndrome. Level of evidence Prospective cohort study, Level II.