Purpose. To examine sedentary behaviours (including television viewing, playing computer games and computer use), diet, exercise and fitness in relation to overweight/obesity in Australian adolescents. Methods. Questionnaires elicited food frequency data, time spent in TV viewing, using computers, other sedentary occupations and physical activity recall. Weight, height and fitness (laps completed in the Leger test) were measured. Results. Among 281 boys and 321 girls, mean age 12 years (SD 0.9), 56 boys (20.0%) and 70 girls (23.3%) were overweight/obese. Greater fitness was associated with decreased risk of overweight/obesity in boys (Odds ratio [OR] 0.74; 95% CI 0.55, 0.99) and girls (OR 0.93; 95% CI 0.91, 0.99). TV-viewing predicted increased risk in boys (OR 1.04; 95'% CI 1.01, 1.06) and decreased risk in girls (OR 0.99; 95% CI 0.96, 0.99). Computer use, video games, and other sedentary behaviours were not significantly related to risk of overweight/obesity. Vegetable intake was associated with lower risk in boys (OR 0.98; 95% CI 0.97, 0.99); greater risk was associated with lower fat intake in boys and girls, lower consumption of energy-dense snacks in boys (OR 0.74; 95% CI 0.62, 0.88) and greater intake of vegetables in girls (OR 1.02; 95% CI 1.00, 1.03), suggesting dieting or knowledge of favourable dietary choices in overweight/obese children. Conclusions. Among these adolescents, fitness was negatively related to risk for overweight/obesity in boys and girls. TV-viewing was a positive predictor in boys and a negative predictor in girls but the effect size was small; other sedentary behaviours did not predict risk.