A 40-year-old woman with a history of injury to the left foot was initially treated for an ankle sprain. When the initial roentgenograms were reassessed, os peroneum fracture with diastasis was diagnosed. This may be the second case of fracture of the uncommon anomaly to be reported in the literature. A fracture of the os peroneum may be confused with ankle sprain. Only careful examination of serial roentgenograms may avoid an incorrect diagnosis and further complications.