We describe the design of a school-based healthy lifestyle intervention for eight-year-oldto nine-year-old school children from lower socio-economic backgrounds designed to10 increase physical activity, decrease sedentary behaviours, reduce screen-time behaviours, encourage healthy attitudes and behaviours to nutrition, and reduce body mass index. The intervention complemented the statutory curriculum and was delivered by aspiring Physical Education teachers in partnership with professional teachers. A non-randomised controlled trial of 416 children aged eight to nine years from 24 schools in Northern 15 Ireland was performed. Schools were randomly assigned to one of two groups, an intervention or control, with 12 schools in each group. The intervention group received the 12-week healthy lifestyles programme based on social cognitive theory. Each week children received a one-hour lesson that included moderate to vigorous intensity physical activity. Lessons were progressive and included follow-up, classroom-based activities. As 20 a motivator to be active, the intervention culminated in a visit to the university for a celebration festival with participants from other schools. At baseline and at 12-week follow-up, physical activity, screen-time behaviours, dietary patterns and self-perceptions were assessed. Anthropometric assessments of height and weight were also taken. Preliminary baseline findings showed no differences on self-report physical activity, and 25 accelerometer-observed vigorous, moderate, light and sedentary behaviour, screen-time behaviours, attitudes to nutrition or self-perceptions. We predict that the intervention will increase physical activity, reduce sedentary behaviour, have a positive influence onnutritional behaviour, reduce screen time, regulate body mass index and increase self perception.With the findings we will provide further information on the effects of a30 school-based intervention to a sample of children from social and financial disadvantage.