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Objective: To study risk factors for overweight among Brazilian adolescents of low-income families.Design: Case–control study of obese and non-obese adolescents.Setting: Anthropometric survey including 1420 students (aged 14–19 years) attending a public high school in São Paulo, Brazil.Methods: Selection of 83 overweight (body mass index (BMI) >85th percentile) and 89 non-overweight (BMI >5th percentile and <85th percentile) subjects, frequency-matched by age, gender, pubertal development and socio-economic status. Among the nutritional, familial and behavioural data available, five covariates (parents' obesity, adolescents' past obesity, to have a best friend, dietary restriction and habit of napping) were included in the fitted hierarchical conditional logistic regression models.Measurements: Parents or guardians and adolescents were weighed, measured and answered a pre-tested questionnaire applied by trained nutritionists and paediatricians.Results: The prevalence of overweight was 15.2%. As previous risks, obese parents and obesity during infancy presented odds ratios (OR) and 95% confidence intervals (CI) of 2.23 (1.15–4.35) and 3.60 (1.47–8.80), respectively. As concurrent factors, the habit of napping, to have a best friend and reported dietary restriction presented OR (95% CI) of 3.43 (1.32–8.92), 5.15 (1.76–15.07) and 7.26 (2.95–17.88), respectively. Dietary patterns, frequency of obesogenic foods and other physical activity indicators presented no statistical significance.Conclusion: In case–control studies, OR may overestimate the true risks. Parents' obesity and previous childhood obesity were identified as risk factors; therefore these factors should be the target for preventive programmes and policies in order to prevent the burden of obesity in the near future. [ABSTRACT FROM AUTHOR]