Objective: To investigate the dietary intake and physical activity of boys and girls aged 9-13 years, and the influence of peers on these behaviours. Design: Cross-sectional study. Setting: Nine primary and secondary schools in south-west London. Subjects: A total of 315 children wore sealed pedometers, provided self-report measures of dietary intake and answered a questionnaire relating to peer influence. Anthropometric measures of height and weight were also obtained. Results: Obese children had the lowest reported energy intakes and the lowest step counts per day. Boys took significantly more steps per day than girls, however girls were closer to achieving their recommended cut-offs for physical activity. Girls had lower energy intakes per day and lower BMI Z-scores than boys, however both genders, across all age groups, had higher than recommended intakes of saturated fat. There were significant associations between peer influence and physical activity levels but not between peer influence and dietary intake. Conclusions: Low energy intake and physical activity levels but high saturated fat intakes among boys and girls across all age groups highlight the importance of promoting both physical activity and healthy food choices. The finding that peers have a significant effect on physical activity levels but not on dietary intake offers an important approach for the design of health promotion interventions and obesity prevention programmes. Such designs may be particularly beneficial for obese youth, since the low physical activity levels found could be a major contributing factor to the maintenance of the condition.