Aims Although of great value to understand the treatment results for mental health problems obtained in clinical practice, studies using naturalistic data from children and adolescents seeking clinical care because of complex mental health problems are limited. Cross-national comparison of naturalistic outcomes in this population is seldomly done. Although careful consideration is needed, such comparisons are likely to contribute to an open dialogue about cross-national differences and may stimulate service improvement. The aim of this observational study is to investigate clinical characteristics and outcomes in naturalistic cohorts of specialized child and adolescent mental health outpatient care in two different countries. Methods Routinely collected data from 2013 to 2018 of 2715 outpatients in the Greater Area of Brisbane, Australia (CYMHS) and 1158 outpatients in Leiden, the Netherlands (LUMC-Curium) were analysed. Demographics, clinical characteristics and severity of problems at start and end of treatment were described, using Children's Global Assessment Scale (CGAS), Health of the Nation Outcome Scales for Children and Adolescents (HoNOSCA) and the parental Strength and Difficulties Questionnaire (SDQ-P). Results Routine outcome measures (CGAS, HoNOSCA, SDQ-P) showed moderate to severe mental health problems at start of treatment, which improved significantly over time in both cohorts. Effect sizes ranged between 0.73-0.90 (CYMHS) and 0.57-0.76 (LUMC-Curium). While internalizing problems (mood disorder, anxiety disorder and stress-related disorder) were more prevalent at CYMHS, externalizing developmental problems (ADHD, autism) prevailed at LUMC-Curium. Comorbidity (>1 diagnosis on ICD10/DSM-IV) was relatively similar: 45% at CYMHS and 39 % at LUMC-Curium. In both countries, improvement of functioning was lowest for conduct disorder and highest for somatoform/conversion disorders and obsessive-compulsive disorders (OCD). Overall, 20-40% showed clinically significant improvement (shift from ...