J. U. Carmona, D. Argüelles, R. Deulofeu, D. Martínez-Puig, M. Prades Veterinary and Comparative Orthopaedics and Traumatology 2009 6 455-459 The aim of this study was to evaluate the clinical and biochemical effects of the administration of oral hyaluronan (Hyal-Joint® [HJ]) on young horses with osteochondrosis (OC). Our hypotheses were that HJ administration is safe, would decrease the degree of synovial effusion and the concentration of nitric oxide (NO) and prostaglandin E2 (PGE2) in synovial fluid, and would increase the concentration of hyaluronic acid (HA) in plasma and synovial fluid. Eleven young horses with tarsocrural OC were included in a randomised, double-blinded, placebo-controlled pilot clinical trial. Six horses received 250 mg/day HJ for 60 days (T60) and five horses received a placebo. The initial values of the degree of synovial effusion, NO, PGE2 and HA concentrations in synovial fluid and HA concentration in plasma were obtained. The horses were evaluated in terms of the same parameters at the end of treatment (T60) and 30 days thereafter (T90). The differences between the groups for each of the parameters evaluated at T0, T60 and T90 were not significant. Nevertheless, the horses treated with HJ tended to show a lower score for synovial effusion as well as higher HA, NO and PGE2 concentrations in synovial fluid, but these differences were non-significant. At a dose of 250 mg/day, HJ did not produce any adverse clinical effects and was well tolerated by the horses