PurposeTo evaluate adolescents' perception of the causes of obesity, with emphasis on differences according to nutritional status and socioeconomic position.MethodsWe conducted qualitative research including 80 adolescents belonging to the 1993 Pelotas (Brazil) Birth Cohort Study, and their mothers. We classified adolescent boys and girls into four groups (girls–obese, girls–eutrophic, boys–obese, and boys-eutrophic) according to body mass index for age and sex, and systematically selected them according to family income at age 15 years. Research techniques included semistructured interviews and history of life. Topics covered in the interviews included early experiences with weight management, effect of weight on social relationships, family history, eating habits, and values.ResultsLow-income obese adolescents and their mothers perceive obesity as a heritage, caused by family genes, side effects of medication use, and stressful life events. However, low-income eutrophic adolescents emphasize the role of unhealthy diets on obesity development. Among the high-income adolescents, those who are obese attribute it to genetic factors and emotional problems, whereas those who are eutrophic mention unhealthy diets and lack of physical activity as the main causes of obesity.ConclusionsPerceptions of the causes of obesity in adolescents from a middle-income setting vary by gender, socioeconomic position, and nutritional status. Whereas some blame genetics as responsible for obesity development, others blame unhealthy diets and lifestyles, and others acknowledge the roles of early life experiences and family traditions in the process of obesity development.