This study investigates the transmission of mental disorders within adolescent peer networks in Finland. Using a population-based registry study, data from Finnish citizens born between 1985 and 1997 were analyzed, focusing on the ninth grade as a critical period for mental disorder onset. The study followed participants from the completion of ninth grade until a diagnosis of a mental disorder, emigration, death, or December 31, 2019, whichever came first. The exposure was having at least one classmate diagnosed with a mental disorder in the ninth grade. The main outcome was the later risk of being diagnosed with a mental disorder. Key findings include: - Among 713,809 cohort members, 47,433 had a mental disorder diagnosis by the ninth grade. - 25.1% of the remaining cohort members (666,376) received a mental disorder diagnosis during follow-up (7.3 million person-years). - A dose-response association was observed, with no significant increase in the later risk for one diagnosed classmate (HR, 1.01; 95% CI, 1.00-1.02), but a 5% increase for more than one diagnosed classmate (HR, 1.05; 95% CI, 1.04-1.06). - The risk was highest during the first year of follow-up, with a 9% increase for one diagnosed classmate (HR, 1.09; 95% CI, 1.04-1.14) and an 18% increase for more than one diagnosed classmate (HR, 1.18; 95% CI, 1.13-1.24). - The risk was greatest for mood, anxiety, and eating disorders. - The association remained significant after adjusting for various confounders, including parental, school-level, and area-level factors. The study concludes that mental disorders may be transmitted within adolescent peer networks, suggesting that prevention and intervention measures should consider peer influences on early-life mental health to reduce the societal burden of mental disorders. Further research is needed to elucidate the underlying mechanisms.This study investigates the transmission of mental disorders within adolescent peer networks in Finland. Using a population-based registry study, data from Finnish citizens born between 1985 and 1997 were analyzed, focusing on the ninth grade as a critical period for mental disorder onset. The study followed participants from the completion of ninth grade until a diagnosis of a mental disorder, emigration, death, or December 31, 2019, whichever came first. The exposure was having at least one classmate diagnosed with a mental disorder in the ninth grade. The main outcome was the later risk of being diagnosed with a mental disorder. Key findings include: - Among 713,809 cohort members, 47,433 had a mental disorder diagnosis by the ninth grade. - 25.1% of the remaining cohort members (666,376) received a mental disorder diagnosis during follow-up (7.3 million person-years). - A dose-response association was observed, with no significant increase in the later risk for one diagnosed classmate (HR, 1.01; 95% CI, 1.00-1.02), but a 5% increase for more than one diagnosed classmate (HR, 1.05; 95% CI, 1.04-1.06). - The risk was highest during the first year of follow-up, with a 9% increase for one diagnosed classmate (HR, 1.09; 95% CI, 1.04-1.14) and an 18% increase for more than one diagnosed classmate (HR, 1.18; 95% CI, 1.13-1.24). - The risk was greatest for mood, anxiety, and eating disorders. - The association remained significant after adjusting for various confounders, including parental, school-level, and area-level factors. The study concludes that mental disorders may be transmitted within adolescent peer networks, suggesting that prevention and intervention measures should consider peer influences on early-life mental health to reduce the societal burden of mental disorders. Further research is needed to elucidate the underlying mechanisms.