Quadrilateral space syndrome is an uncommon injury. The true prevalence is unknown because of a lack of literature and possible misdiagnosis. Prevalence may increase as knowledge of the syndrome increases. The case is presented of a recreational triathlete who had a spontaneous onset of quadrilateral space syndrome. The diagnosis was made by physical examination and confirmed with magnetic resonance imaging. A conservative, yet aggressive rehabilitation programme resulted in functional improvement within six weeks. Results have been maintained for eight weeks.