PURPOSE: The genes in myopia twin study were established to assess the relative genetic contribution of spherical equivalent using a classical twin model. This manuscript will provide a detailed outline of the methodological design, twin recruitment, and the prevalence of myopia in the genes in myopia twin study. METHODS: All Victorian-based twins registered with the Australian Twin Registry aged 18 years or older were invited to participate genes in myopia twin study. Each subject underwent a general questionnaire, comprehensive eye examination, and a blood sample was collected. Myopia was defined as worse than or equal to -0.50 diopters sphere (in at least one eye). RESULTS: A total of 627 twin pairs out of 4,158 twin pairs consented to participate in the genes in myopia twin study. A total of 345 monozygotic and 267 dizygotic twin pairs aged between 18 and 86 years were examined. The response rate for monozygotic twins (19.8%) was almost double that of dizygotic twins (11.7%). The overall prevalence of myopia was 29.7% for all twins. CONCLUSIONS: The genes in myopia twin study is the first Australian-based twin study to assess refraction in an adult twin population and the largest of its kind in the world. The comprehensive testing protocol used in the in the genes in myopia twin study has provided an extensive twin database for genetic analysis. Participation rate was found to vary according to zygosity, gender, and age.