BACKGROUND: The aim of this study was to investigate overweight and obesity among a representative population of 15 966 Norwegian 15-16 year olds and the associations with different socio-economic and cultural risk factors. METHODS: Self-reported data were obtained from school-based surveys in six counties during 2000-04. Overweight and obesity were calculated using Cole's index. RESULTS: The prevalence of overweight and obesity were 11.8% and 2.4%, respectively, higher among boys. Logistic regression analyses revealed that adolescents in Nordland, Troms and Finnmark (the northernmost counties) were 70-90% more likely to be overweight and obese compared with adolescents in Oslo (the capital and southernmost county) (OR for overweight in Finnmark = 1.7, CI = 1.3, 2.3). Lower educational plans and poor family economy were both significantly associated with overweight and obesity. So was physical inactivity (OR = 1.2, CI = 1.1, 1.3 and OR = 1.6, CI = 1.2, 2.1, respectively). Eating breakfast was positively associated with not being overweight/obese. CONCLUSION: Overweight and obesity is associated with socio-economic factors and with factors related to food habits and nutrition, suggesting important areas for prevention.