Primary lens luxation is the most common form of lens displacement encountered in the dog in the United Kingdom. The condition arises in early to late middle age, is essentially bilateral and occurs typically in the terrier breeds. Subluxation is rarely observed before 3 years of age but luxation of the lens is ultimately almost invariable. In Tibetan Terriers bred for the condition zonular changes were evident at least as early as 14 months of age. The condition could serve as a model for late spontaneous dislocations of the lens in man.