A case of concordance for unilateral congenital ptosis in monozygotic twins is presented. The literature on genetics and congenital ptosis is reviewed. A heritability index is calculated for congenital ptosis, based on all current twin data including the present report. A value for the heritability index of 0.75 is found, suggesting a strong hereditary influence for congenital ptosis. We conclude that twin data strongly support a transmissable genetic defect as contributing to the development of congenital ptosis.