The results of arthroscopic surgery in the treatment of osteochondral fragmentation of the proximodorsal aspect of the first phalanx and the influence of other fetlock joint lesions on prognosis were evaluated in 336 horses. Horses were classified as: 1) returning to previous use at the same or higher class of performance; 2) returning to previous use (regardless of class of performance); or 3) failing to return to previous use. Ninety-six horses (29%) had fragmentation alone; 140 horses (42%) had fragmentation and additional fetlock lesions, and 100 horses (29%) underwent concurrent carpal arthroscopy. Of the 100 horses that underwent carpal arthroscopy, 63 had proximodorsal first phalanx fragmentation alone and 37 had other fetlock lesions associated with the fragment. There was significant association between lesion type and return to previous use for the Thoroughbred racehorse group. There was also a significant association between lesion type and return to the same or higher class of racing for the Thoroughbred racehorse group. No significant association in return to previous use existed for racehorses vs. non-racehorses, Thoroughbred racehorses vs. Quarter Horse racehorses, single vs. multiple joint involvement, and single vs multiple fragmentation per joint. The Thoroughbred racehorses in this study were sensitive to additional fetlock or carpal lesions, as was exemplified by the significant effect lesion type had on outcome.