We describe a patient who sustained a displaced isolated intra-articular fracture of the distal ulna, causing limitation of rotation of the forearm. The extent of displacement of the fracture which was not evident on plain radiographs was revealed by CT. The fracture was reduced and internally fixed using a standard technique applicable to the fixation of fractures of the radial head. Full movement was restored. An isolated injury to the distal ulna is rare and requires careful clinical and radiological assessment.