Purpose: Localization of the lacrimal sac is a critical step during endoscopic dacryocystorhinostomy (endo-DCR). A "light pipe" can be used to transilluminate the lacrimal sac endonasally. We hypothesized that this may misguide the surgeon learning endo-DCR to create an osteotomy mostly posterior to the maxillary line if only the bone overlying the transillumination was to be removed, as the thinner lacrimal bone will transmit light more readily than the thicker maxillary bone of the frontal process of the maxilla that forms the anterior lacrimal sac fossa.Methods: The charts of 32 patients with primary acquired nasolacrimal duct obstruction in whom a lighted system was used during endo-DCR at Massachusetts Eye and Ear from April 2015 through October 2016 were reviewed. Patients with prior history of lacrimal surgery or trauma directly to the lacrimal sac fossa were excluded. Location of the maximal point of transillumination in relation to the maxillary line was observed and noted intraoperatively.Results: Of a total of 39 endo-DCR surgeries performed, the intraoperative transillumination point was entirely posterior to the maxillary line in 32 instances (82%).Conclusions: Use of an endocanalicular light pipe preferentially illuminates posterior to the maxillary line endonasally. The anterior lacrimal sac fossa (maxillary line and anterior as visualized endonasally) is rarely transilluminated, likely due to thicker bone in that region. Surgeons learning how to perform endo-DCR using a light pipe should be aware of this phenomenon.