The hereditary cases of microphthalmia in the mentally retarded are not all of the partially sex-linked type as has hitherto been stated. A survey is given of published cases of dominant, autosomal recessive and sex-linked recessive pedigrees with the coincidence of microphthalmia and mental retardation. The common misconception of Norrie's disease being regraded as microphthalmia or hereditary corneal dystrophy instead of phthisis is noted. Present knowledge of chromosomal aberrations associated with microphthalmia is surveyed, and some of the syndromes and phenocopies in the mentally retarded in which microphthalmia may be found are reviewed. The survey, however, does not pretend to be a complete inventory of causes of microphthalmia in the mentally retarded.