Objective: To study prevalence and factors associated with mental health service use among 18-year-old adolescent boys. Method: Predictors at age 8 and factors at age 18 associated with mental health service use during the preceding 12 months were studied in a general population sample of 2,316 Finnish boys born in 1981 attending military call-up (79% of the original sample). Results: Within the preceding 12 months, 2.1% of the boys had used mental health services. At age 18, internalizing, anxious-depressive, and withdrawal symptoms; health problems; not living with parents; use of illicit drugs; high level of alcohol use; and regular smoking were independently associated with service use. At age 8, a high level of emotional and behavioral symptoms, need for referral, and low school performance according to teacher evaluations predicted service use 10 years later. Conclusions: Only a minority of adolescents with severe problems had used mental health services. Because of the wide range of problems and comorbidity among service users, there is a need for integration of different services. Education services have a central role in the early detection of those who will later use mental health services. J. Am. Acad. Child Adolesc. Psychiatry, 2004;43(10):1250-1258. Key Words: mental health, service use.