Objectives Employing national registers for research purposes depends on a high diagnostic validity. The aim of the present study was to examine the diagnostic validity of recorded diagnoses of early-onset obsessive-compulsive disorder (OCD) in the Danish Psychiatric Central Register (DPCR). Design Review of patient journals selected randomly through the DPCR. Method One hundred cases of OCD were randomly selected from DPCR. Using a predefined coding scheme based on the Children's Yale Brown Obsessive Compulsive Scale (CYBOCS), experienced research nurse or child and adolescent psychiatrists assessed each journal to determine the presence/absence of OCD diagnostic criteria. The detailed assessments were reviewed by two senior child and adolescent psychiatrists to determine if diagnostic criteria were met. Primary outcome measurements Positive predictive value (PPV) was used as the primary outcome measurement. Results A total of 3462 children/adolescents received an OCD diagnosis as the main diagnosis between 1 January 1995 and 31 December 2015. The average age at diagnosis was 13.21±2.89 years. The most frequent registered OCD subcode was the combined diagnosis DF42.2. Of the 100 cases we examined, 35 had at least one registered comorbidity. For OCD, the PPV was good (PPV 0.85). Excluding journals with insufficient information, the PPV was 0.96. For the subcode F42.2 the PPV was 0.77. The inter-rater reliability was 0.94. The presence of the CYBOCS in the journal significantly increased the PPV for the OCD diagnosis altogether and for the subcode DF42.2. Conclusion The validity and reliability of International Classification of Disease 10th revision codes for OCD in the DPCR is generally high. The subcodes for predominant obsessions/predominant compulsions are less certain and should be used with caution. The results apply for both children and adolescents and for both older and more recent cases. Altogether, the study suggests that there is a high validity of the OCD diagnosis in the Danish National Registers.