Available evidence indicates that rates of childhood overweight and obesity have been increasing over the past two decades, but inconsistencies between study methods moderate the strength of this evidence.

Concomitant health problems and associated costs make it imperative that primary prevention initiatives are introduced to combat the obesity epidemic.

Fundamental to informed action is anthropometric monitoring, which if properly implemented will identify changes over time in specific populations to inform policies, practices and services aimed at prevention and treatment.

Sample representativeness is essential for valid trend and prevalence data, but efforts to obtain population-based anthropometric data from school children with the required written parental consent have been thwarted by low participation rates.

Notable improvements in participation rates when utilising opt-out consent, in which participation is assumed unless otherwise indicated, are evident from local as well as international studies. Opt-out consent can facilitate anthropometric monitoring, delivering a more informed, best-value-for-money response to the obesity epidemic.

Health and education ethics committees need to acknowledge the benefits of opt-out consent for “low-risk” anthropometric measurement, which ultimately upholds the individual’s rights.