Coronoid process fractures are reported to occur from avulsion by the brachialis muscle or to be associated with elbow dislocations. We report a rare case of coronoid process fracture due to avulsion by the anterior bundle of the medial collateral ligament rendering the elbow unstable. In children, small fracture fragments of the coronoid process (types 1 & 2) are in reality often much larger but the actual size is not appreciated radiographically, as the coronoid process contains considerable amounts of cartilage. If the fragment is seen to be significantly displaced it may have resulted from avulsion by important structures such as the medial collateral ligament and open reduction is required to stabilise the elbow.