Birth order is considered one of the most influential environmental factors in child development, affecting cognitive abilities and behavioral traits. This study investigates the effect of birth order in relation to attention-deficit hyperactivity disorder (ADHD), the most common neuro-behavioral disorder of childhood. The study describes birth order of 598 children aged 6 to 18 years diagnosed due to attention-deficit hyperactivity disorder. The cohort contains relatively large size families because 47.1% of the participants were born in families of more than 4 children. The results show no statistically significant differences in birth order of children among all families. We conclude that the chances of first, middle, or later born children, as well as single children, to suffer from attention-deficit hyperactivity disorder are almost equal. This study provides evidence that birth order has no effect in relation to attention-deficit hyperactivity disorder.