<p>Effective October 2015, all US health care institutions and practitioners transitioned to the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) coding system, raising concerns about the validity of examining trends over time in clinical care, costs, and quality. For child mental health disorders, alignment with psychiatric diagnosis groups in the Diagnostic and Statistical Manual of Mental Disorders (Fifth Edition) (DSM-5) is also required to consistently examine trends. This is because the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) and ICD-10-CM diagnosis codes differ for individual disorders, and child mental health disorders are reclassified under new psychiatric diagnosis groups. We thus developed the Child and Adolescent Mental Health Disorders Classification System (CAMHD-CS), which classifies child mental health disorders across coding systems and aligns with DSM-5 psychiatric diagnosis groups. To examine our system’s performance, we compared detection of these disorders in a Medicaid claims database against the Clinical Classification Software (CCS) groups and then examined identification of the disorders across ICD-9-CM and ICD-10-CM coding systems using a national pediatric hospitalization database.</p>