The study of behavioural factors associated with nutritional status in adolescence is relevant for analysing the upward trend in overweight among adolescents in Brazil. A total of 759 adolescents from the public and private schools in the Pinheiros district of São Paulo were studied. Data was collected on weight, height, food consumption and physical activity. Overweight was classified according to Body Mass Index (BMI) values. The association between overweight and characteristics of food consumption or physical activity was stratified by gender and school network. The prevalence of overweight was 25.2 per cent (men 27.3 per cent; women 23.8 per cent). Overweight was positively associated with diet and inversely associated with the consumption of unhealthy foods in the school network and the consumption of healthy foods in public schools. There was no association between being overweight and skipping/replacing meals or practising physical activity inside or outside school. High prevalences of overweight were found among the adolescents studied. There is a need for actions to promote healthy behaviours in the social group assessed.