Os trigonum impingement is a frequent cause of posterior ankle pain in ballet dancers because they need extreme plantar flexion during the execution of relevé in demipointe and en pointe positions. Clinical examination and standard and modified x-rays should be carried out to clearly identify the site and entity of the impingement. If a posterior impingement is clinically diagnosed, standard and modified magnetic resonance imaging should be also performed. From September 2005 to September 2006, we considered 186 young trainee ballet dancers. Twelve suffered from posterior ankle pain, and six of these had os trigona. We treated all the ballet dancers nonoperatively, and nine of them had good results. Conservative treatment failed in only three cases after 1-4 mos of physical and medical therapies, and, in these cases, good results were obtained through surgical excision of the accessory ossicle. Modified x-ray and magnetic resonance imaging help to determine the site and entity of the posterior impingement. If this is properly diagnosed, good results can be obtained through a nonoperative approach in a majority of cases.