Background: The Child and Adolescent Service Intensity Instrument is a tool to determine the appropriate level of care placement for a child or adolescent. The CASII links a clinical assessment of the child and its environment with standardized levels of care using a detailed algorithm. It can be used for children aged 6 to 18 years with psychiatric disorders, substance use disorders, or developmental disorders. This study reports on the translation of the instrument into Dutch and tests of its validity and reliability using both case vignettes and real-life cases seen within the Belgian mental health and child welfare systems. Methods: Reliability testing of the CASII was conducted based on both standard vignettes and live cases. To test the validity, the CASII was compared to the Children's Global Assessment Scale (CGAS) and the Strengths and Difficulties Questionnaire (SDQ). Trained professionals with various education and active in different sectors completed the CASII. Results: Using case vignettes, the intra-class correlation coefficients for the different dimensions ranged from .29 to .79. The ICC for placement recommendations was excellent (.74). The ICCs for the subscale ratings for the live cases ranged from .40 to .90. The CASII showed good validity when compared to the CGAS (correlations ranging between .47-.82). When compared to the SDQ, the CASII correlated low to moderate with the total difficulties score (.05-.37) but correlated slightly higher with the impact score (.25-.35). Conclusion: The data confirm the usefulness of the CASII among different service providers of different sectors with a broad range of clinical experience and professional training. The findings extend and partially replicate other findings and suggest reasonable but not unequivocal validity and reliability across linguistically and culturally different contexts.