A large national study of schoolchildren aged 6-18 years was conducted to assess nutritional and socio-cognitive factors associated with body mass index (BMI). A questionnaire was used to assess nutritional quality of breakfast, importance of physical activity and food variety score, among 4441 students from randomly selected schools in all states and territories of Australia between September and December 2000. Height and weight were measured. Nutritional knowledge, dietary self-efficacy and dietary locus of control were also assessed among adolescents. School socio-economic status (SES) was derived from parental income. The factors were modelled using multiple linear regression to determine significant predictors of BMI. Dietary self-efficacy, nutritional quality of breakfast and SES were found to be the principal predictors of BMI in addition to the expected biological factors of age, gender and height. Furthermore, low SES was found to contribute to high BMI, mediated by the low nutritional quality of breakfast. Food variety was positively associated with high BMI and this was mediated by dietary self-efficacy. Nutrition knowledge and dietary locus of control were not associated with BMI. These results suggest that breakfast programmes for low-income children may be an effective measure in the prevention of childhood obesity. (Contains 2 tables and 1 figure.)