Objective: To study associations between comorbid psychopathology and long-term outcomes in a large birth cohort sample from age 8 to early adulthood. Method: The sample included long-term outcome data on 2,556 Finnish boys born in 1981. The aim was to study the impact of early childhood psychopathology types (externalizing versus internalizing versus both) and informant sources (self-report versus parent/teacher reports) on young adult outcomes, based on data from a military registry of psychiatric diagnosis, a police registry on criminal and drug offenses, and self-reported problems in late adolescence and early adulthood. Results: Children with combined conduct and internalizing problems at age 8 had the worst outcomes and highest risk of subsequent psychiatric disorders, criminal offenses, and self-reported problems at follow-up, with 62% of these boys manifesting psychiatric disorders, committing criminal offenses, or both at follow-up. Although these children included only 4% of the sample, they were responsible for 26% of all criminal offenses at follow-up. In contrast, children with conduct problems without internalizing problems and those with attention problems had much less severe but nonetheless elevated levels of risk of antisocial personality disorder and criminal offenses. Long-term outcomes for these two groups were substantially better than for children with combined conduct and internalizing problems. Children with "pure" emotional problems had an elevated risk only of similar emotional problems at follow-up. Conclusions: The subjective suffering and long-term burden to society is especially high among children with comorbid conduct and internalizing problems in childhood. A major challenge for child and adolescent psychiatric, education, and social services is to develop effective intervention strategies focusing on these children. Additional longitudinal epidemiological studies of this comorbidity group are needed, and, if replicated, such findings will have important implications for future diagnostic classification systems (DSM-V). (Contains 2 tables.) [This study was supported by the Sigrid Juselius Foundation, Yrjo Jahnsson Foundation, and Signe and Arne Gyllenberg Foundation.]