Objective: The sinus tympani (ST) comprise one of the most hidden areas in the Human body. It is one of the most common locations of residual cholesteatomas and is in close proximity with tie facial nerve and stapes. These characteristics render ST as a key factor in chronic otitis media surgeries. This study aimed to investigate the clinical features and intraoperative findings of cholesteatoma patients with and without ST invasion (STI).,Methods: One hundred and fifty-one cholesteatoma patients who had undergone the canal wall-down procedure at our center were retrospectively reviewed. They were categorized into two groups: cholesteatoma patients with and without STI. Comparisons were made between the two groups in terms of the disease duration, surgical technique, rate of facial canal dehiscence (FCD), and number of locations of FCD and erosion of tie stapes suprastructure. The mean hearing gain of the patients who underwent hearing reconstruction was compared between both groups.,Results: The rates of disease duration >5 years, radical -nastoidecto-ny surgery, and erosion of tie stapes suprastructure were significantly nigher in patients with STI than in those without STI. Mean hearing gain was significantly higher in patients wit-lout STI than in those with STI. The numbers of locations and rate of FCD were also significantly higher in patients with STI than in those without STI.,Conclusion: The presence of STI in cholesteatoma patients is a significant intraoperative finding for the predicting the extent of FCD. STI should serve as a warning to surgeons because it indicates a potential for less functional outcomes due to erosion of the stapes suprastructure.