The records of 469 institutionalised mentally retarded subjects were examined. Keratoconus and blindness were present in 8 per cent and 4 per cent of the ninety-one subjects with Down syndrome. Only one case of keratoconus was found among the 378 subjects with mental deficiency of other aetiology. The incidence of strabismus, nystagmus and cataract was also increased in subjects with Down syndrome, especially in females. Cases of acute keratoconus associated with Down syndrome reported in the world literature are summarised. A total of about forty cases has ben reported between the ages of nine and fifty-three years. Sight has been preserved equally well after both conservative management and keratoplasty in eyes with and without cataracts.