A. Berkley Chesen, DVM, DACVS; Robin M. Dabareiner, DVM, PhD, DACVS; M. Keith Chaffin, DVM, MS, DACVIM; G. Kent Carter, DVM, MS, DACVIM Journal of the American Veterinary Medical Association June 1, 2009 234 11 1432-1436 Objective—To determine clinical signs, ultrasonographic findings, and outcome of horses with tendinitis of the proximal portion of the superficial digital flexor tendon (SDFT; group A horses) and to compare signalment, horse use, and outcome in these horses with that of horses with tendinitis of the midmetacarpal region of the SDFT (group B horses). Design—Retrospective case series. Animals—12 group A horses and 22 group B horses. Procedures—Medical records were reviewed for signalment, horse use, and outcome information; data for group A horses also included severity of lameness, diagnostic results, and treatment. Results—Most group A horses were aged (mean, 18 years; median, 17 years; range, 11 to 23 years) Quarter Horses (9/12 horses) and had a grade 3 or 4 (on a scale from 1 to 5) forelimb lameness. Most group A horses (8/12 horses) had positive reactions to carpal flexion and were (9/12 horses) sound following ulnar nerve blocks. Ultrasonographic evaluation revealed hypoechoic SDFT lesions (median echogenicity score, 3/4; median fiber alignment score, 3/3; and mean length, 9.75 cm). Group A horses were significantly older and had a poorer outcome for return to previous use (2/12 horses), compared with group B horses (median age, 5 years; 10/22 horses returned to previous use). Thirteen of 22 group B horses were Thoroughbreds. Conclusions and Clinical Relevance—Tendinitis of the proximal portion of the SDFT was a cause of lameness in aged performance horses; prognosis for return to previous use was poor.