Collateral ligament injury is usually the result of trauma or a fall. Lameness varies from mild to severe, depending on the severity of injury. Flexion of the hock is usually resented and will exacerbate lameness. Effusion of the tarsocrural joint and periarticular swelling are evident. Lameness may be improved by intra-articular analgesia of the tarsocrural joint. Diagnosis is confirmed with ultrasonography. Increased uptake of radionuclide at the proximal and distal attachments of the damaged ligament may be evident on scintigraphy, and enthesiopathy may be apparent radiologically in horses with chronic injury. Conservative treatment is recommended, with physical and chemical anti-inflammatory treatments in the acute stages and then a period of stall rest (4–6 mo) followed by a controlled ascending exercise program. Prognosis depends on the severity of injury.