Effect of intraarticular injections of hyaluronic acid was evaluated in two groups of horses, using force plates to assess quantitatively the degree of lameness observed. In six horses, chip fractures were created surgically on the dorsomedial aspects of both radial carpal bones. After the horses were trained for 30 days, 40 mg of hyaluronic acid was injected intraarticularly into the limb showing the greater degree of lameness. Horses were worked for 2 more weeks and then were evaluated. A highly significant (P less than 0.01) increase in weight bearing on the treated limb was observed. Ten horses with clinical unilateral front limb lameness, involving a single joint, also were treated with 40 mg of hyaluronic acid. The day after the injection, the horses resumed training and were examined 2 weeks later. At that time, the horses were free from observable lameness, and this observation was corroborated by force plate data. According to their trainers, the horses' performance improved markedly after the injection and remained improved. Seemingly hyaluronic acid was beneficial when injected into the arthritic joints of these two groups of horses.