REASONS FOR PERFORMING STUDY: Hyaluronan (HA) has been used to treat joint disease via intra-articular, i.v. and oral administration. The efficacy of intra-articular and i.v. use has been evaluated but the oral route has yet to be examined. OBJECTIVES: To determine the effect of oral hyaluronan gel on joint effusion following arthroscopic surgery for osteochondritis dissecans (OCD) of the tarsocrural joint of yearling Thoroughbreds. METHODS: Forty-eight yearlings diagnosed with unilateral or bilateral osteochondritis dessicans (OCD) of the tarsus were arbitrarily chosen prior to arthroscopic surgery. The yearlings were included only if they had mild or no synovial effusion pre-surgery. Twenty-four of the yearlings (27 joints) were treated with 100 mg of HA orally for 30 days post operatively and 24 (30 joints) with a placebo orally for 30 days. At 30 days post operation, a blinded examiner scored the effusion of the dorsomedial tarsocrural joint individually using a scale of 0 to 5 (0 = no effusion, 1 = barely palpable effusion, 2 = palpable effusion [without plantar effusion], 3 = golf ball sized effusion with plantar effusion, 4 = tennis ball sized effusion with plantar effusion, 5 = > tennis ball sized effusion with plantar effusion). Half grades were allowed and OCD lesion sizes and locations were compared. RESULTS: A total of 57 joints were examined, of which 33 had OCD of the distal intermediate ridge of the tibia, 19 OCD of the distal lateral trochlear ridge of the talus and 5 OCD of the medial malleolus. The mean 30 day effusion score of the HA treated group (27 joints) was 0.67 while the mean of the 30 day placebo group (30 joints) was 2.05 (P < or = 0.0001). Similar results were noted when comparing treated vs. placebo for each lesion location as well as for lesion sizes. CONCLUSIONS AND POTENTIAL RELEVANCE: Oral preparations of hyaluronan are being used to treat joint disease in horse. Anecdotal reports supporting the efficacy of these preparations already exist. This study provides objective evidence that oral HA reduces joint effusion post operatively following the arthroscopic removal of an OCD lesion in the tarsocrural joint.