The study comprised of 10 patients (nine ballet-dancers and one athelete) who underwent excision of the os trigonum, due to pain on full plantar-flexion. In each case, at surgery the retrotalar fat appeared to be under pressure and bulged through the incision site. Histological examination revealed extensive fibrosis of this fat. It is postulated, therefore, that impingement had occurred between the posterior lip of the tibia and the plantar flexed calcaneum. A subsequent ballet-dancer (not included in the nine), who experienced retrotibial pain, had no os trigonum and whose bone scan was positive, underwent simple adipose tissue excision, with equally good results. A similarity is noted with Hoffa's syndrome and the question has arisen as to whether it is necessary to excise the os trigonum or whether removal of the fat pad alone will suffice.