Purpose: To assess the relative influence of genetic and environmental factors on optic disc size and cup/disc ratio in healthy eyes. Methods: A sample of 55 monozygotic and 50 dizygotic healthy twin pairs aged 20-46, all having the same sex within pairs (47 pairs were male) had optic discs measured from colour fundus photographs according to the Wisconsin Protocol. Structural equation modelling was used to estimate the relative contribution of genetic and environmental factors to the phenotype. Results: Disc dimensions did not vary significantly by age or sex. After adjusting for age and sex, additive genetic factors (i.e. heritability) explained 77% (95% CI: 65-85%) of variation of vertical disc diameters, whereas estimated unshared environmental effect was 23% (95% CI: 15-35%). For vertical cup diameters, heritability accounted for 70% (95% CI: 55-80%) and environmental factors 30% (95% CI: 20-45%). For cup/disc ratio, additive genetic and unshared environmental factors explained 66% (95% CI: 48-77%) and 34% (95% CI: 23-52%) of the variations, respectively. Discussion: In this healthy twin sample, we found that three quarters of the variations in vertical optic disc and optic cup diameters were attributable to genetic influence.