Preventing firearm violence requires understanding its antecedents. Yet no comprehensive longitudinal study has examined how involvement with firearms during adolescence-use, access, and victimization (defined as threatened with a weapon or gunshot injury)-is associated with the perpetration of firearm violence in adulthood. To examine the association between firearm involvement during adolescence and subsequent firearm perpetration and ownership in adulthood among youth involved in the juvenile justice system. This cohort study analyzed interview responses of 1829 randomly selected participants as part of the Northwestern Juvenile Project, a longitudinal study of health needs and outcomes of youth sampled from a temporary juvenile detention center in a large US city. Youth aged 10 to 18 years were interviewed in detention from November 1995 through June 1998. Participants were reinterviewed up to 13 times over 16 years through February 2015, for a total of 17 776 interviews. The sample was stratified by sex, race/ethnicity, age, and legal status (juvenile or adult court). Data were analyzed from April 2017, when data preparation began, through November 2020. Firearm involvement during adolescence: use (ie, threaten, shoot), access (ownership, ease of access, firearm in household, membership in gang that carries firearms), and victimization (gunshot injury, threatened with a weapon). Firearm involvement during adulthood: perpetration of firearm violence (ie, threatening with or using a firearm) and firearm ownership. Among the 1829 participants, 1388 had a 16-year follow-up interview: 860 males, 528 females; 809 were African American, 203 were non-Hispanic White; 374 were Hispanic; and 2 were other race/ethnicity; median (interquartile range) age of 32 (30-32) years. Eighty-five percent of males and 63.2% of females were involved with firearms as adolescents. Compared with females, males had significantly higher odds of every type of involvement except having a firearm in the home. In adulthood, 41.3% of males and 10.5% of females perpetrated firearm violence. Adolescents who had been threatened with a weapon or injured by firearms had 3.1 (95% CI, 2.0-4.9) and 2.4 (95% CI, 1.2-4.9) times the odds of perpetrating violence during adulthood. Similar associations were found for firearm ownership. Involvement with firearms during adolescence-including victimization-is a significant risk factor for firearm perpetration and ownership during adulthood.