Record linkage of the Finnish Twin Cohort Study with the Hospital Discharge Registry and with the Registry of Rights to Free Medication kept by the Social Insurance Institution gave following numbers of twin pairs with an ophthalmic disease (one or both members of the pair had the disease) in each disease category: 98 with glaucoma simplex (5 concordant pairs), 38 with capsular glaucoma (no concordant pairs), 58 with iritis (no concordant pairs) and 149 with strabismus (2 concordant pairs). The number of concordant pairs in each disease category was small, except for glaucoma simplex in which concordant pairs could be broken down by zygosity. The ratio of observed to expected (based on association by chance) was 6.96 for MZ and 1.74 for DZ pairs. This result suggests that genetic factors play some role in the variability of prevalence of simple glaucoma. Main etiologic factors are still to be found in the environment. Data on occurrence of other diseases of ophthalmologic importance is presented.