The Growing Up in Australia study, funded by the Department of Family and Community Services through the Stronger Families and Communities Strategy, is being developed as Australia's first national longitudinal study of children. This paper outlines the proposed sample design for the study, including details of the sampling frame, the sample selection process and expected sample size and sample loss. A multiple cohort cross-sequential design has been proposed, with one cohort comprising children aged under 12 months, and the other children aged 4 years. The sample will be selected from the Medicare database, the most comprehensive database of Australia's population. A clustered design, based on postcodes, has been chosen as it allows community level effects to be measured and analysed and also allows for reasonably cost effective face to face interviewing.