Deviations from expected perfect symmetry on biological forms can occur, and organisms develop several kinds of asymmetries. Among others there are fluctuating asymmetry and directional asymmetry (DA). DA happens whenever one side on the plane of symmetry develops more than the other side. To study the presence of DA in the horse, we obtained 11 paired linear measurements on 19 mandibles belonging to 'Cavall Pirinenc Català' horse breed, a local breed from Catalonia, NE Spain. The results confirm the existence of asymmetry for certain traits, demonstrating the existence of DA. This is suspected to be caused by a masticatory preference to the left side, as it has been described for other animals. Although further investigation is needed to clarify the clinical relevance of such findings in the horse, it follows that examination and recording of chewing side preference for domestic mammals merits more consideration. The measurements presented here can also be used as a reference for researchers designing experimental studies, especially on mandibular catch-up growth, and as an aid for zooarchaeologists comparing results from dead animals with those from living equine populations.