An unusual case is reported of accidental extrusion of sodium hypochlorite solution into the maxillary sinus during instrumentation in the course of root canal treatment of a maxillary premolar. This caused a burning sensation accompanied by nasal bleeding, swallowing of blood, and interrupted breathing. The case was managed by irrigating with sterile saline, arresting nasal bleeding, and administering antibiotics, anti-inflammatory drugs, and decongestants. Schlagwörter: endodontic irrigating solution, maxillary sinus, sodium hypochlorite