Subcutaneous rupture of the peroneus longus tendon occurring through a fracture of the os peroneum (OP) or secondary to posterior dislocation of this uncommon sesamoid bone is rare. The authors report three incidents of this injury, which presented either as fracture of the OP with diastasis of the fragments or as posterior dislocation of the whole OP without fracture. In all cases conservative treatment yielded satisfactory results. It is emphasized that, in all cases of ankle injury, radiologists should carefully examine the plain radiographs because most OP injuries are not seen in images obtained at the time of injury and are discovered retrospectively when the fragments separate. Not treating the problem at an early stage may lead to instability of the ankle or to peroneal compartment syndrome.