Objectives. To test for socioeconomic differences in some biological and behavioral risk factors for obesity in a representative and contemporary sample of adolescents. Methods. Cohort study of 2 016 randomly selected 12- and 15-year-olds representative of Northern Ireland, studied in 2000. We tested for differences in obesity risk factors based on a priori hypotheses between adolescents from affluent (n=487) versus deprived (n=237) families. Potential risk factors were dietary energy and macronutrient intake, habitual physical activity, TV viewing and computer use, and physical fitness. Results. Adolescents of higher socioeconomic status reported significantly lower habitual energy intake (210 kJ/kg/d SD 80 vs. 229 kj/kg/d SD 91, p <0.01); significantly higher levels of habitual physical activity (physical activity score 25.9 SD 16.6 vs. 20.9 SD 16.4, p <0.001), and had significantly higher cardiorespiratory fitness (estimated VO2 max 46.2 ml/kg/min SD 8.4 vs. 43.4 ml/kg/min SD 8.3, p <0.001). Prevalence of overweight and obesity (BMI >= 85(th) percentile) in the cohort was 29.1% and was slightly but not significantly higher in the low (33.8%) versus the high (28.5%) socioeconomic groups. Conclusions. Differences in some of the biological and behavioral risk factors for obesity exist between adolescents of different socioeconomic status in Northern Ireland. These may help explain the basis of established socioeconomic differences in obesity risk.