Objective: The associations of birth order, number of siblings, and ADHD was examined. Method: The analysis based on representative, epidemiological data from the German Health Interview and Examination Survey for Children and Adolescents (KiGGS) study (N = 13,488). Results: An increased risk for ADHD in firstborn versus youngest born children (odds ratio [OR] = 1.31, 95% confidence interval [CI] [1.09, 1.58]) and also versus children with no sibling (OR = 1.31, 95% CI [1.03, 1.68]) was revealed, while number of siblings was not associated with ADHD. Results remained stable after controlling for confounders. Conclusion: Firstborn children may receive simultaneously less parental resources and more responsibilities if younger siblings are born. This happens during the vulnerable developmental period of ADHD. In addition, due to higher levels of insecurity, parents are assumed to focus more on potential physical or psychological abnormities in their firstborn children. This may result in a diagnostic bias in firstborn children.