Lens opacities, including cataracts, are frequent findings during routine ophthalmic examination in the horse, and clinicians often experience difficulty in predicting the potential progression and consequences on visual function of these opacities. This has particular relevance to the prepurchase examination. The present study proposes a descriptive classification of the more common lens opacities observed in the horse based principally on their morphological origin. This classification provides a basis for objective prediction of the probable progression of lens opacities, in particular cataracts, in the horse.