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PURPOSE To evaluate the genetic contribution in age-related macular degeneration (ARMD) by a disease-ascertained twin study. METHODS Concordance rates for ARMD in 25 twins were obtained by using four masked graders to confirm the diagnosis of ARMD and place subjects in one of three categories; concordant, intermediate, or discordant. Demographic features and known risk factors for ARMD were compared between monozygotic and dizygotic twin pairs. RESULTS Of the 25 twin pairs, 15 were monzygotic and 10 were dizygotic. All 15 monozygotic twins were concordant or intermediate for ARMD. Of the dizygotic twin pairs, only one was concordant and five were discordant. In the demographic and risk factor analysis no unusual contributing or confounding variables were detected. CONCLUSIONS The association between zygosity and concordance for ARMD suggests a major importance for genetics in the etiology of ARMD. Our data further support a multi-factorial, primarily polygenic etiology for the condition.