In this study, the long-term outcome and the development of osteoarthritis (OA) after collateral ligament (CL) rupture in metacarpophalangeal (MCP) or metatarsophalangeal (MTP) joints with either open or closed MCP / MTP joints was evaluated. Horses were included in the study on the basis of radiographic evidence of subluxation or luxation in stressed MCP / MTP joints in the dorsopalmar / dorsoplantar (DP) view. Horses were clinically and radiographically evaluated at first presentation in the clinc as well as at follow-up examination. Fifteen horses met the inclusion criteria. Lameness varied between 2 / 5 to non-weightbearing lameness. Three horses presented with a clinically severely instable fetlock. Opening of the MCP / MTP joint as consequence of associated wounds or lacerations was diagnosed in 4 horses. In 11 horses CL rupture without opening of the associated MCP / MTP joint was diagnosed (no wounds communicating with the MCP / MTP joint). Six of these horses were treated conservatively, 5 were treated surgically. Three months after admission 11 horses were sound at walk and trot. Three horses showed a grade 3-4 / 5 lameness. One of these was euthanatized 6 weeks later because severe OA had developed in the injured MTP joint. Another horse had been euthanized because of laminitis. In 12 horses long-term follow-up examination (9 months – 12 years) was possible. All horses showed radiographic signs of OA in the affected MCP / MTP joint. In 6 horses there was also mild to moderate OA in the PIP joint of the affected limb. 67% (n = 10) of the horses returned to be used for pleasure riding as prior to injury, 13% (n = 2) of the horses remained pasture sound, 20% (n=3) were euthanatized because of persistent lameness. The study shows the long-term prognosis of CL ruptures with open and closed MCP / MTP joints. In horses with chronic lamness after CL rupture in MCP / MTP joint the PIP joint should also be considerd as a potential source of pain.