Background Attention-deficit-hyperactivity disorder (ADHD) has been found to be related to overweight/obesity in children and adolescents, but it is a heterogeneous disorder, and the relationships between the dimensions of ADHD and overweight/obesity are not clear. The aim of this study was to explore which dimensions of the disorder are specifically associated with overweight/obesity. Methods The study sample consisted of 300 treatment-naive children with ADHD and 75 healthy controls aged 7-17 years. The ADHD module of the Kiddie Schedule for Affective Disorders and Schizophrenia-Present and Lifetime Version was used to diagnose ADHD. The severity of ADHD symptoms was assessed via Conners' Parent Rating Scale (CPRS). The weight, height, and breast-feeding duration of the study samples and controls were recorded. Body mass index (BMI) was categorized according to the national age/sex-specific reference values. Results The rate of overweight/obese children was higher in the ADHD group. The association between ADHD symptoms and BMI percentile scores was evaluated using structural equation modeling. In that model, it was observed that the Cognitive Problems/Inattentive and Oppositional subscores of the CPRS had a positive predictive effect on the BMI percentile scores, but breast-feeding duration had a negative predictive effect on the BMI percentile scores. Conclusion Inattention, oppositionality and breast-feeding duration were associated with overweight/obesity in children and adolescents with ADHD. Longitudinal studies are needed to more fully understand this relationship and the mechanisms underlying the association between ADHD and overweight/obesity.