Over-strain injury of the superficial digital flexor tendon (SDFT) is a common injury in the horse. Tendon appears to adapt to loads placed on it during development, but fatigue damage accumulates after skeletal maturity, which is inadequately repaired and predisposes to clinical tendinitis. In any population of horses, there is a wide variation in SDFT mechanical properties. A noncollagenous protein, cartilage oligomeric matrix protein (COMP), is particularly abundant during growth in the equine SDFT and has been proposed to have an organisational role in the formation of collagenous matrices. This study aimed to determine whether COMP levels were correlated to mechanical properties at skeletal maturity. Tendons from 2 groups of 12 horses were analysed: Group 1 horses with restricted age, 2 years +/- 2 months, showed a significant correlation between both ultimate tensile stress modulus of elasticity and stiffness and COMP, while Group 2 mature horses with varying age did not, because of age- and exercise-induced loss of COMP. These data supports the hypothesis that COMP is an important mediator in the growth of tendon. This data would suggest that the identification of low COMP levels in tendon during growth would indicate horses prone to tendon injury and methods of promoting COMP synthesis during growth would potentially improve tendon quality and reduce the risk of subsequent tendinitis.