Patient selection is important for any successful surgical procedure. Adenotonsillectomy for obstructive sleep apnoea secondary to adenotonsillar hypertrophy is no exception. This retrospective Taiwanese study attempts to find preoperative factors that have a positive influence for this commonly performed operation. A total of 63 children (age 2-16) with an Apnoea/Hypopnoea Index (AHI) of more than 1 per hour on preoperative polysomnography underwent adenotonsillectomy. Surgical success (42/63 patients – 66.7%) was declared if there was an improvement of more than 50% on the AHI three months postoperatively. Preoperative factors evaluated included age, BMI, tonsil size, adenoid size, cephalometry, obesity and AHI. A multivariate logistic regression analysis was used to assess the relationship between these preoperative factors and surgical success. The only statistically significant relationships were between tonsil size and preoperative AHI. This study supports the notion that the bigger the tonsils one removes, the better the chance of improving the child’s OSA. Also, the higher the preoperative AHI, the higher the chance of surgical success. Based on these results, if a child has large tonsils and/or severe AHI an adenotonsillectomy has a higher chance of improving their OSA. This data will no doubt be helpful when consenting such patients in the future.