We report the intraoperative use of polyvinyl siloxane impression material to demonstrate the anatomy of the lacrimal sac, canaliculi, and lacrimal duct in a case of congenital lacrimal sac fistula. A 1-week-old boy was examined for tearing since birth. Examination revealed a left congenital lacrimal sac fistula. After a failed surgery to close the fistula with silicone intubation at 6 months of age, the patient underwent endonasal dacryocystorhinostomy performed at 14 months of age, aided by intraoperative injection of polyvinyl siloxane (trade name Reprosil) to mark and protect the nasolacrimal sac and facilitate endonasal visualization. A polyvinyl siloxane cast demonstrated the anatomy of the accessory canaliculus causing nasolacrimal duct obstruction. Postoperatively, the epiphora resolved and the fistula remained closed. The polyvinyl siloxane cast provides a 3-dimensional "ex vivo " model of the lacrimal sac, upper duct, and canalicular anatomy, and can be used in dacryocystorhinostomy surgery to identify and protect the lacrimal sac.