Computed tomography (CT) scans of the paranasal sinuses act as roadmaps for endoscopic sinus surgery (ESS) and careful inspection preoperatively warns the operating surgeon of critical anatomical variants. This study aimed to investigate if implementation of a pre-ESS CT checklist using the ‘CLOSE’ mnemonic increases the number of anatomical variants correctly identified by ENT trainees. CLOSE stands for: Cribriform (Keros classification); Lamina papyracea- (dehiscence/orbital prolapse); Onodi/Haller cells (present/absent); Sphenoid (optic/carotid dehiscence or hypoplastic) and Ethmoid (identify anterior ethmoidal arteries). Eighteen ENT residents in a US institution participated in the study. The supervising surgeon documented if the trainee correctly identified up to 10 potential CT findings via a short interview just prior to surgery. The CLOSE checklist covered most of the 10 findings apart from three: concha bullosa; type III/IV frontal cell; and osteogenesis. Twenty-eight pre-implementation and 29 post-implementation scans were reviewed. The study showed a significant improvement in the percentage of correctly identified anatomical variants, 24% versus 84%, using the checklist. I found this study useful as a reminder for systematic evaluation of pre-ESS CT scans and also in providing evidence to support the use of checklists to potentially reduce surgical complications.