OBJECTIVES: To examine ethnic differences in body mass index (BMI), food habits and physical activity, and determine the factors contributing to differences in BMI. DESIGN AND METHOD: In 2000-2001, 7343 (response rate 88%) 15- and 16-year-old students, enrolled in lower secondary schools in Oslo, participated in the cross-sectional Oslo Health Study. Of these participants, 1719 were defined as ethnic minorities. RESULTS: Significant gender and ethnic differences in mean BMI were observed. Of the ethnic minority adolescents, 5.8% were underweight (85th percentile of the US CDC/NCHS reference distribution). BMI was not significantly associated with either socio-economic factors or physical activity. Food habits and physical activity differed with ethnicity but not with socio-economic factors. An ordinal regression showed that girls from East Asia (odds ratio (OR) 0.4) and boys from sub-Saharan Africa (OR 0.4) had lower BMI than the Western group. Among girls, higher BMI was associated with less frequent consumption of chocolates and sweets, full-fat milk and breakfast (OR 2.4, 1.7 and 1.7, respectively). Higher BMI, for both boys and girls, was associated with current and past dieting (OR 3.7 and 4.2, respectively). CONCLUSIONS: Adolescent food habits and physical activity varied by gender and ethnicity but not with socio-economic factors. BMI was associated with ethnicity, gender and food habits, but no significant relationship was observed with socio-economic factors or physical activity. Ethnicity, in addition to gender, should be taken into consideration when studying BMI and associated factors among adolescents.