A population-based study in Finland examined whether having classmates with a mental disorder diagnosis in ninth grade was associated with later risk of being diagnosed with a mental disorder. The study included 713,809 individuals born between 1985 and 1997, followed from age 16 until diagnosis, emigration, death, or December 31, 2019. The results showed a dose-response association, with a 5% increased risk for having more than one diagnosed classmate. The risk was highest in the first year of follow-up, with a 9% increase for one diagnosed classmate and 18% for more than one. The risk was greatest for mood, anxiety, and eating disorders. After adjusting for various confounders, the association remained significant. The study suggests that mental disorders may be transmitted within adolescent peer networks. However, the study has limitations, including potential residual confounding and the crude measure of peer networks through school classes. Further research is needed to clarify the mechanisms underlying the possible transmission of mental disorders.A population-based study in Finland examined whether having classmates with a mental disorder diagnosis in ninth grade was associated with later risk of being diagnosed with a mental disorder. The study included 713,809 individuals born between 1985 and 1997, followed from age 16 until diagnosis, emigration, death, or December 31, 2019. The results showed a dose-response association, with a 5% increased risk for having more than one diagnosed classmate. The risk was highest in the first year of follow-up, with a 9% increase for one diagnosed classmate and 18% for more than one. The risk was greatest for mood, anxiety, and eating disorders. After adjusting for various confounders, the association remained significant. The study suggests that mental disorders may be transmitted within adolescent peer networks. However, the study has limitations, including potential residual confounding and the crude measure of peer networks through school classes. Further research is needed to clarify the mechanisms underlying the possible transmission of mental disorders.