Medical records of 70 horses diagnosed with left laryngeal hemiplegia that were treated by use of prosthetic laryngoplasty combined with bilateral ventriculectomy were examined. Degree of arytenoid cartilage abduction that had resulted from the prosthesis had been graded before the horse was discharged from the hospital. Follow-up information through owner/trainer questioning was acquired for 55 horses. Information concerning the type and frequency of complications, if any, was recorded, along with a determination of owner/trainer satisfaction regarding surgical results. Race records were acquired for 42 horses and were used to aid in evaluation of postoperative performance. After surgery, Thoroughbred racehorses had a much lower success rate (48%; 19 of 40 horses), determined from owner/trainer questioning, than did all breeds in the study that were not intended for racing (93%; 14 of 15 horses). Younger Thoroughbred race-horses had a better success rate (70%; 14/20) than did those > or = 3 years old (25%; 5/20). Degree of arytenoid abduction had little effect on outcome, except that horses with maximal abduction (grade 5) of the arytenoid cartilage had a higher prevalence of complications and poor correction. The 2 most prevalent complications were exercise intolerance (42%; 23 of 55 horses) and continuing noise when exercising (47%; 26 of 55 horses).