OBJECTIVE: To study risk factors associated with overweight and obesity among adolescents enrolled in private high schools in the city of Pelotas, southern Brazil. METHOD: This was a case-control study. The subjects were 264 overweight (body mass index (BMI) > or =85th percentile of the National Center for Health Statistics (NCHS) reference population) and 264 non-overweight (BMI >5th and <85th NCHS percentiles) adolescents identified by means of an anthropometric survey which included 1608 students. The adolescents were weighed, measured and interviewed about food habits and physical activity at school. Parents provided their own weights and heights when contacted by telephone, and parental BMI was calculated. RESULTS: Hierarchical multiple conditional logistic regression showed that overweight and obesity were positively associated with mother's (odds ratio (OR) 2.86, 95% confidence interval (Cl) 1.43-5.74) and father's (OR 2.43, 95% Cl 1.37-4.30) BMI >/=30 kg m(-2), being overweight before 10 years of age (OR 2.26, 95% Cl 1.30-3.90) and the habit of dieting (OR 3.53, 95% Cl 1.76-7.22). Having more than three meals per day was found to be a protective factor against overweight and obesity (OR 0.54, 95% Cl 0.29-1.00). CONCLUSION: The present study showed that a family history of obesity, overweight during childhood and the habit of dieting for weight-loss purposes are factors associated with obesity during adolescence. The habit of having more than three daily meals turned out to be a protective factor against overweight. These results suggest the necessity for early intervention at the family and general community levels aimed at the prevention of obesity through actions directed towards the modification of established behaviours.