Objective: To determine the effect of child-, parent- and family-related factors on the use of and need for mental health services by 12 years of age. Method: A prospective population-based questionnaire study of 1,287 first-born children was launched in Finland in 1985. The Child Behavior Checklist (CBCL) was completed by 1,086 parents when the children were aged 3 years. At the age of 12, the CBCL, the Youth Self-Report, and other potential determinants of service need and use were obtained from 908 parents and 900 children (80% response rate). Results: Of the total sample, 7.2% had used professional services, and 3.3% had needed services without obtaining them. Elevated scores on the age 3 CBCL Total Problems and Externalizing scale predicted independently later service use but not perceived need of services. In cross-sectional data at age 12, parental ratings of the child's problem behavior were associated with both the need for and use of services. Poor social competence and parental distress were independently associated with use of services. Conclusions: Recognizing behavior problems in early childhood and evaluating family stress factors are important in social and health care systems designed for children. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(8):974-983. Key Words: epidemiology, mental health services, Child Behavior Checklist.