Heterogeneity of microphthalmos and coloboma is considerable and the clinical morphology is insufficient for etiological classification. A nosological classification has been performed in 86 cases of microphthalmos and coloboma occurring in mentally retarded individuals. Although nearly all were sporadic cases, a precise diagnosis could be established in 45 per cent of the cases. In 13 per cent, the phenotypes could not be assigned to any similar malformation syndrome described in the literature, and in 42 per cent the precision of the classification was low although the malformations had been repeatedly described before. An unknown number of the last-mentioned cases may have been environmentally induced, but little is known of environmentally induced ocular malformations and how to diagnose them after the first year of life.