BACKGROUND: The os trigonum is a secondary center of ossification of the talus localized lateral to the groove for the flexor hallucis longs. Usually it remains asymptomatic but symptoms like pain and swelling may arise from activities requiring repetitive forced plantar flexion. Treatment is initially conservative with rest, icing, use of nonsteroidal anti-inflammatory drugs producing good results. When non-operative treatment fails to decrease the symptoms of posterior ankle impingement syndrome, surgical endoscopic excision of the os trigonum is the treatment of choice.OUR EXPERIENCE: We evaluated all patients who underwent excision of a symptomatic os trigonum by posterior endoscopic technique with a mean follow up of three years. All patients had preoperative lateral radiographs in maximum plantar flexion and magnetic resonance imaging to find eventual associated lesions of the ankle and the subtalar joint. The patients were evaluated pre and postoperatively according to American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot score and the Tegner score.RESULTS: The clinical evaluation and the scales used showed a statistically significant improvement of the function and symptoms.CONCLUSION: The endoscopic excision of the symptomatic os trigonum is a safe and effective procedure with a reduced risk of complications, short recovery period, and good clinical outcome.