Ankle diastasis without associated fracture occurs in a latent form in which the diastasis is detected only by stress radiographs, and in a frank form with the diastasis visible on routine, unstressed radiographs. Whereas latent ankle diastasis requires no reduction and can be treated by cast immobilization, frank diastasis requires anatomical reduction of the ankle mortise. The method of reduction depends upon the particular type of frank diastasis. We have identified four types of frank ankle diastasis without fracture. Type I injuries demonstrate straight lateral fibular subluxation without plastic deformation of the fibula and are best treated by open reduction, removal of any interposed soft tissue, and stabilization with a tibiofibular screw. Type II injuries present with straight lateral subluxation of the fibula due to plastic deformation of the distal fibula and may require a fibular osteotomy for reduction prior to internal fixation. Plastic deformation of the fibula as a cause of ankle diastasis has not been previously reported. The uncommon type III injury consists of posterior rotatory subluxation of the fibula. In type IV injuries the talus is dislocated superiorly, resulting in divergence of the tibia and fibula. Type III and IV injuries can usually be treated by closed manipulation and plaster immobilization. The authors treated four type I and two type II patients by open reduction and internal fixation. Both type II injuries required fibular osteotomy to restore the normal tibiofibular relationship. Good results were obtained in four patients. Fair results secondary to stiffness and pain on activity were present in two patients. All patients maintained anatomical reduction of the ankle mortise following removal of the tibiofibular screw.