Description Objective: To examine changes in the prevalence of six key chronic disease risk factors (the “Big 6”), from before (2019) to during (2021) the COVID-19 pandemic, among a large and geographically diverse sample of adolescents, and whether differences over time are associated with lockdown status and gender. Design: Prospective cohort study. Location: New South Wales, Queensland and Western Australia. Participants: 983 adolescents drawn from the Health4Life Study.