Importance  Bullying and being exposed to bullying among children is prevalent, especially among children with psychiatric symptoms, and constitutes a major concern worldwide. Whether childhood bullying or exposure to bullying in the absence of childhood psychiatric symptoms is associated with psychiatric outcomes in adulthood remains unclear.

Objective  To study the associations between bullying behavior at 8 years of age and adult psychiatric outcomes by 29 years of age.

Design, Setting, and Participants  Nationwide birth cohort study of 5034 Finnish children with complete information about childhood bullying behavior was followed up from 8 to 29 years of age. Follow-up was completed on December 31, 2009, and data were analyzed from January 15, 2013, to February 15, 2015.

Main Outcomes and Measures  Information about bullying, exposure to bullying, and psychiatric symptoms were obtained from parents, teachers, and child self-reports when children were 8 years of age. Use of specialized services for psychiatric disorders from 16 to 29 years of age was obtained from a nationwide hospital register, including outpatient and inpatient treatment.

Results  Among the 5034 study participants, 4540 (90.2%) did not engage in bullying behavior; of these, 520 (11.5%) had received a psychiatric diagnosis at follow-up; 33 of 166 (19.9%) who engaged in frequent bullying, 58 of 251 (23.1%) frequently exposed to bullying, and 24 of 77 (31.2%) who both frequently engaged in and were frequently exposed to bullying had received psychiatric diagnoses at follow-up. When analyses were adjusted by sex, family factors, and child psychiatric symptoms at 8 years of age, we found independent associations of treatment of any psychiatric disorder with frequent exposure to bullying (hazard ratio [HR], 1.9; 95% CI, 1.4-2.5) and being a bully and exposed to bullying (HR, 2.1; 95% CI, 1.3-3.4). Exposure to bullying was specifically associated with depression (HR, 1.9; 95% CI, 1.2-2.9). Bullying was associated with psychiatric outcomes only in the presence of psychiatric problems at 8 years of age. Participants who were bullies and exposed to bullying at 8 years of age had a high risk for several psychiatric disorders requiring treatment in adulthood. However, the associations with specific psychiatric disorders did not remain significant after controlling for concurrent psychiatric symptoms.

Conclusions and Relevance  Exposure to bullying, even in the absence of childhood psychiatric symptoms, is associated with severe adulthood psychiatric outcomes that require treatment in specialized services. Early intervention among those involved in bullying can prevent long-term consequences.