AbstractBackground:ADHD is a highly prevalent disease in childhood which often persists into adulthood, then co-occurring with common adult conditions. Especially for adult ADHD, little is known about the costs of ADHD and the additional costs of comorbid conditions.Aims:To determine medical costs of ADHD and costs of comorbidities (mood, anxiety and substance use disorders, obesity), including their co-occurrence rates, stratified by age and gender.Method:Claims data from a German Statutory Health Insurance database with approximately four million member-records per year were analysed. A total of 25,300 prevalent ADHD patients were identified by means of an ICD-10 GM diagnosis of ADHD. A 1:1 age and gender adjusted reference group without ADHD diagnosis was randomly selected. Total health claims and health care costs related to ADHD were analysed, in addition to more targeted analyses of the occurrence and costs of pre-defined common comorbidities of, in particular, adult ADHD (SUD, mood and anxiety disorders, obesity). Outcomes were mean costs per patient and occurrence rates of comorbid conditions. Surplus costs of a comorbid condition in persons with ADHD relative to costs of this condition in persons without ADHD were calculated. Subgroup analyses were conducted based on age (0–12 years, 13–17 years, 18–30years, 30+ years) and gender.Results:Patients with ADHD were €1500 more expensive annually than individuals without ADHD (p < 0.001). Main cost drivers were inpatient care, psychiatrists and psychotherapists. Mood, anxiety, substance use disorders and obesity were significantly more frequent in ADHD patients and additional costs resulting from the comorbid conditions amounted up to €2800. Costs were slightly higher in women than men and increased with age for both genders. In young adults (18–30 years) health care costs dropped notably, especially costs for the medical treatment of ADHD with stimulants and costs for psychiatrists, before rising again in the group of patients over 30 years who had higher comorbidity rates.Conclusions:Medical costs for ADHD are substantial, in part through frequently occurring comorbid conditions, and particularly in adulthood, and are likely to further accelerate in the coming years. A gap of care was found, starting with the transition age group of patients over 17 years, as indicated by reduced costs per person during young adulthood, as well as an overall strong drop in administrative prevalence. In the future, approaches to improve the situation of care and reduce costs at the same time, i.e. through managed care programmes, should be implemented and benefit from detailed knowledge on age and gender-specific cost-drivers.