The location of the maxillary os and its importance to the endoscopic sinus surgeon. As functional endoscopic sinus surgery continues to gain popularity and support, the necessity for a clear and accurate understanding of the anatomy of the ostiomeatal complex becomes essential. To clarify this anatomy, serial cadaver dissections were performed and the anatomy of the ostiomeatal complex was detailed in three dimensions, with an emphasis on precise localization of the internal os of the maxillary sinus as it relates to the orbit, natural antronasal canal, and ethmoid infundibulum. Measurements of the position of the internal os relative to the position of the anterior and posterior walls of the maxillary sinus and the position of the orbit were taken. The dimensions and configuration of the antronasal canal and its relationship to the infundibulum were also detailed. These measurements and relationships must be understood for an endoscopic sinus surgeon to locate the natural ostia without injuring the orbit. 