Objective: To describe the psychometric properties and test the reliability of a new instrument designed to measure mental health services use within pediatric clinical samples, the Services for Children and Adolescents-Parent Interview (SCAPI), which was developed by the National Institute of Mental Health Multimodal Treatment Study of Children With Attention Deficit Hyperactivity Disorder (MTA). Method: Similarities and differences with other measures of services use are described. Ten types of services are measured by the SCAPI. Formal test-retest reliability testing was carried out in 104 subjects with a mean time between tests of 18 days. Results: Test-retest [kappa] values ranged from 0.49 to 1.00, with an overall [kappa] value for all services of 0.97. Seven of the 10 service types had [kappa] values of 0.75 or higher, indicating excellent reliability. In addition, matched responses on specific questions about reasons for seeking services, starting and ending dates, number and length of visits, and type of provider seen were more than 75% for most service categories, consistently so for reporting of medications and school services. Conclusions: The SCAPI is a reliable instrument for assessing mental health and related services use and may be an especially valuable adjunct in studies involving clinical samples, especially clinical trials. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(11): 1345-1354. Key Words: mental health services, instruments, services research.