The age distribution in Iran is changing; soon we will face an ageing population.1,2 Non-communicable diseases such as cardiovascular diseases, cancer, diabetes and osteoporosis more commonly affect adults and older people.3 These diseases are associated with a range of risk factors, such as smoking, hypertension, hypercholesterolaemia, inadequate physical activity and overweight/obesity, that are mostly preventable.4 Also, older people are more likely to experience frailty, loneliness, depression/emotional distress, economic insecurity/poverty, functional limitation/disability and need for help for daily activities.5–11 Many studies have been conducted in Iran to examine different aspects of heath in the older population. However, most of them have been cross-sectional, focused on a specific feature of ageing, small in sample size or conducted in a non-representative sample of older adults, weaknesses that make it difficult to generalize findings or translate them into policy. Cohort studies among older adults in Iran are scarce.12,13 Those studies that exist either focus on a specific outcome (e.g. Busher cohort study including 3000 people in the south of Iran, focusing on cardiovascular diseases) or include a relatively small population (e.g. Amirkola ageing study in the north of Iran with a sample size of ∼ 1200 people). Moreover, these studies only include individuals aged 60 years and over, which does not allow us to look at how factors in late mid-life, e.g. employment, might affect later life. In order to assess the dynamic ageing process, it is necessary to include people from middle age, e.g. 45 or 50 years old onwards. This is in line with other cohort studies of older adults, e.g. the Health and Retirement Study in the USA, the China Health and Retirement Longitudinal Study (CHARLS), and the Canadian Longitudinal Study on Aging (CLSA). Therefore, there is a need for a comprehensive and multidisciplinary longitudinal study to understand better the older population’s health, well-being and health care needs in Iran. The Prospective Epidemiological Research Studies in IrAN (PERSIAN) is a nationwide cohort study launched in 2014 to bring together research in the fields of medicine, epidemiology, health and nutrition.14 The PERSIAN Cohort is the first national prospective cohort study that aims to include 200 000 men and women between 35 and 70 years of age from 18 sites throughout the country, focusing on non-communicable diseases and their associated risk factors [www.persiancohort.com]. They are being followed up for 15 years.