Objective: ADHD is one of the most common childhood psychiatric disorders. Research indicates that there is some link between obesity/overweight and ADHD, though the mechanism of this association remains uncertain. It is the aim of the present study to explore the association between ADHD, obesity, and plasma leptin, neuropeptide Y (NPY), and adiponectin levels. Method: Thirty-six patients diagnosed with ADHD were included in the study. The control group consisted of 40 healthy children and adolescents who had similar age and gender features with the patient group. Plasma leptin, adiponectin, NPY levels were measured, and body mass index (BMI), weight for height, and standard deviation scores (SDS) of height, weight, and BMI were calculated. Results: No significant difference was found between patients and healthy children in terms of BMI and BMI percentile. Participants were classified into three groups according to their weight to height values. There was no significant difference between the two groups, but 10% of the control group and 30.6% of the ADHD group were classified as overweight, which was 3 times higher than the control group. The adiponectin plasma level was significantly lower and leptin/adiponectin (L/A) ratio was significantly higher in the ADHD group. There was no significant difference between serum NPY levels. In the ADHD group, the mean leptin plasma level was high, but was not statistically significant. Conclusion: We think that a low adiponectin level and high L/A ratio may be the underlying mechanism of the obesity in ADHD patients.