This study examined the full spectrum of comorbid disorders in all statutory-health-insured children aged 5 to 14 years with ADHD in 2017 by using nationwide claims data in Germany. Children with ADHD (n = 258,662) were compared for the presence of 864 comorbid diseases with a control group matched by gender, age, and region of residence (n = 2,327,958). Among others, metabolic disorders (odds ratio [OR] = 9.18; 95% confidence interval [CI] = [8.43, 9.99]), viral pneumonia (OR = 4.95; 95% CI = [2.37, 10.33]), disorders of white blood cells (OR = 4.55; 95% CI = [3.83, 5.40]), kidney failure (OR = 3.33; 95% CI = [2.65, 4.18]), hypertension (OR = 3.26; 95% CI = [3.00, 3.55]), obesity (OR = 2.85; 95% CI = [2.80, 2.91]), type 2 diabetes (OR = 2.61; 95% CI = [2.11, 3.23]), migraine (OR = 2.49; 95% CI = [2.37, 2.61]), asthma (OR = 2.19; 95% CI = [2.16, 2.22]), atopic dermatitis (OR = 2.10; 95% CI = [2.16, 2.23]), juvenile arthritis (OR = 1.56; 95% CI = [1.39, 1.76]), glaucoma (OR = 1.51; 95% CI = [1.30, 1.75]), and type 1 diabetes (OR = 1.30; 95% CI = [1.20, 1.40]) were more likely to be diagnosed in ADHD children. Along with psychiatric diseases, various somatic diseases were more common in ADHD children. The results have direct implications for patient care, including fine-grained diagnostics and personalized therapy.