OBJECTIVES:  Coexisting mental health disorders are common in patients with ADHD and may increase utilization of health care services and therefore expenditures.  The present study was designed to address the impact of comorbidities on the direct medical costs incurred by statutory health insurance (SHI) for patients with and without ADHD.  METHODS:  For a retrospective matched cohort study, concomitant diagnoses and health care resource utilization data for patients age 7 to 19 years with a diagnosis of ADHD (F90.0 and/or F90.1) and for a randomly selected control group (matched 1:1 by age and gender) were extracted from the Nordbaden claims database (for year 2003), and were combined with SHI prescription data.  Complete datasets were available for 2171 children age 7-12 years and 768 adolescents age 13-19 years with a diagnosis of ADHD, plus the same number of control persons.  For costing, resource use was valued applying SHI acquisition costs.  Patient subgroups were defined by the (additional) presence of the most prevalent comorbidities, i.e., conduct and personality disorders, mood and affective disorders, specific development disorders, and adjustment disorder.  RESULTS:  Average costs per patient with ADHD were €622/€661 (children/adolescents) compared to €245/€250 for controls.  ADHD with coexisting conditions caused the following direct medical expenditures:  in the additional presence of conduct and personality disorder, €703/€769; mood and affective disorders, €714/€761; specific development disorders, €630/€766; adjustment disorder, €829/€963.  Average costs for patients with these disorders but without ADHD were also increased, and will be reported in detail.  CONCLUSIONS:  The present data are limited since they do neither include costs of inpatient treatment nor cost of ergotherapeutic interventions, which will have to be addressed in future studies.  They provide nevertheless, for the first time, insight into the impact of coexisting conditions on the financial burden for the SHI associated with a diagnosis of ADHD.