The GOS began as a population-based study designed to investigate the epidemiology of osteoporosis in Australia. The cohort was recruited to address the need for definitive data on the prevalence of osteoporosis, to describe age-related changes in bone mineral density, and to characterise the risks for osteoporosis and fracture. Initially, the GOS comprised only women; men were recruited later. Individuals selected at random from the electoral roll were mailed a letter of invitation with a request to return a response slip or telephone the research centre at Barwon Health (The Geelong Hospital).