A 5-year old Swiss warmblood gelding was admitted to the Veterinary Surgery Clinic of the University of Zurich with a history of lameness in both hind limbs. The site of the lameness was located by intraarticular anesthesia at the tibiotarsal joints. Radiographically two small fragments where diagnosed at the dorsal aspect of the intermediate sagittal ridge of the distal tibia and bony proliferations at the medial proximal tubercle on both tibiotarsal bones. Furthermore the horse showed signs of bone spavin at the distal intertarsal joints of both limbs. The conservative treatment consisting of an intra articular injection of a hyaluronan and a steroidal antiinflammatory drug was not successful in abolishing the lameness. Because of the guarded prognosis and other reasons, the owners decided to put the horse down. The post mortem examination showed large bony fragments with an irregular surface at the proximal end of the medial trochlear ridge of the right and the left talus. On both sides the medial proximal tubercle of the tibiotarsal bone was too small compared to a normal tarsal bone. These bony fragments showed signs, which can be found in osteochondrosis, in separate ossification, in cases of direct trauma and avulsion fractures. Combining the findings of the anamnesis, the clinical, radiographic and post mortem examination, an avulsion fracture of the medial proximal tubercle of the tibiotarsal bone in both rear limbs was diagnosed.