Background: Tower Hamlets is a socioeconomically disadvantaged borough, home to the UK’s largest South Asian population, a group at increased risk of obesity-related diseases. Previous studies in this population have reported high levels of adiposity and inactivity. No borough-wide study has been conducted objectively measuring physical activity patterns. This study aimed to investigate pedometer-determined activity levels of Tower Hamlets' schoolchildren, their association with adiposity and differences according to ethnicity and socioeconomic status (SES). The study was preceded by reviews investigating the association between step counts and adiposity in children and investigating the validity of pedometers as a measure of physical activity in young people. Methods: Participants were recruited from Tower Hamlets' secondary schools (n=884; 584 boys, 300 girls). A pedometer was worn for 7 days. Internationally recognised mean daily step count cut-offs (boys = 15000, girls = 12000) were used to define activity level. Body mass index (BMI), bioelectrical impedance analysis (BIA)-determined percentage body fat (%bf) and waist circumference (WC) were all measured. Children were classified as being of normal weight, overweight or obese according to international cut-off points. A questionnaire was administered to establish socioeconomic status and ethnicity. Results: A total of 884 schoolchildren were recruited (66% boys, 34% girls). Of this, 657 (74%) provided a full set of pedometer, anthropometric and socio-demographic data. Sixty-five percent of all participants were South Asian and 55% received free school meals. Significant differences in anthropometric variables were observed according to gender, ethnicity and school. The prevalence of overweight/obesity ranged widely for boys (35%, 53% and 65%) and girls (33%, 55% and 55%) according to BMI, %bf and WC, respectively. The majority of participants provided 4 or 5 days of activity data, with 15% providing data for 7 days. Inactivity was high, 83% of boys and 72% of girls failed to meet the minimum recommended daily step counts. Activity was greater during the week compared to the weekend and those that were most active during the week were also more active at the weekend. Boys (11580±3560) took significantly more steps than girls (10062±3239) and differences were also observed between schools. No significant differences in activity levels were observed according to ethnicity, SES or adiposity levels. Conclusion: The vast majority of schoolchildren in Tower Hamlets fail to reach current physical activity recommendations, irrespective of ethnicity or socioeconomic class. Inactivity is greater at the weekend. The prevalence of overweight/obesity is also higher than national averages. Intervention strategies to increase physical activity and tackle overweight/obesity in this cohort are required.