BACKGROUND: To ensure their protection and healthy development, children exposed to adverse family circumstances might be placed in foster homes, institutions, or kinship care (out-of-home placement). We aimed to compare the rates of psychiatric diagnoses and criminal convictions in young adulthood (ages 18-25 years) among children who were first placed at ages 2-6 years with those of children who were not placed and who had similar sociodemographic and family characteristics. METHOD: We did a population-wide cohort study using the 1987 Finnish Birth Cohort, which collects longitudinal data linking nationwide child welfare, medical, and criminal registers for all 59 476 livebirths in Finland in 1987. The exposure was the first out-of-home placement at ages 2-6 years. Outcomes were rates of psychiatric diagnoses, criminal convictions, and prescriptions for psychotropic medication filled at ages 18-25 years. We matched cases to non-placed controls using propensity score matching on parental characteristics (eg, age, psychiatric diagnoses, education, family structure) and child characteristics (eg, neurodevelopmental problems, prematurity). Differences in adult outcomes between children placed and matched controls were assessed by use of logistic regression on the matched cohort. FINDINGS: Of 54 814 individuals with complete data, 388 (1%) were first placed at ages 2-6 years; matched controls were identified for 386 of these children. At ages 18-25 years, those who had been placed as children had greater odds than never-placed controls of substance-related disorders (odds ratio 2.10, 95% CI 1.27-3.48), psychotic or bipolar disorders (3.98, 1.80-8.80), depression or anxiety (2.15, 1.46-3.18), neurodevelopmental disorders (3.59, 1.17-11.02), or other disorders (2.06, 1.25-3.39). Participants who were placed had more psychotropic medication prescriptions (1.96, 1.38-2.80) and higher rates of criminal convictions (violent offences, 2.43, 1.61-3.68; property offences, 1.86, 1.17-2.97). INTERPRETATION: Preschool children who are placed out-of-home are at risk of adverse outcomes as adults, even accounting for their initial circumstances. It is important to explore which conditions lead to more or less favourable outcomes in child protection. FUNDING: Academy of Finland.< Leer menos