rom 1999 to 2013, an average of some 15 000 teenagers 12 to 19 years old died annually in the United States. The 3 leading causes of death among teenagers were unintentional injuries (45%), homicide (14%), and suicide (13%). 1 Among these fatal youth injuries, most homicides were gunrelated (83%), and about half of suicides involved a gun (45%). 2 When adolescents resolve their interpersonal conflicts with guns, serious or fatal injuries are likely consequences. 3 There is a strong consensus in the United States that unsupervised youth should not carry guns in public. The federal Gun Control Act of 1968 prohibits gun dealers from selling handguns to youth (<21 years) and selling rifles and shotguns to those younger than 18 years. A 1994 amendment further determined 18 years as the national minimum age for purchasing or possessing handguns or handgun ammunition. Many state laws include age-specific restrictions on the purchase, possession, or storage of firearms. Research on youth-focused gun laws is limited and has shown mixed results. A study 4 showed that state child access prevention (CAP) laws were associated with a modest reduction in suicide rates among youth aged 14 to 17 years, whereas minimum age restrictions for the purchase and possession of firearms do not seem to reduce overall rates of suicide among youth. Even for CAP laws, a study found that the aggregate benefits across IMPORTANCE Gun violence and injuries pose a substantial threat to children and youth in the United States. Existing evidence points to the need for interventions and policies for keeping guns out of the hands of children and youth. OBJECTIVES (1) To examine the association between state gun law environment and youth gun carrying in the United States, and (2) to determine whether adult gun ownership mediates this association. DESIGN, SETTING, AND PARTICIPANTS This was a repeated cross-sectional observational study design with 3 years of data on youth gun carrying from US states. The Youth Risk Behavior Survey comprises data of representative samples of students in grades 9 to 12 from biennial years of 2007, 2009, and 2011. We hypothesized that states with more restrictive gun laws have lower rates of youth gun carrying, and this association is mediated by adult gun ownership. EXPOSURES State gun law environment as measured by state gun law score. MAIN OUTCOMES AND MEASURES Youth gun carrying was defined as having carried a gun on at least 1 day during the 30 days before the survey. RESULTS In the fully adjusted model, a 10-point increase in the state gun law score, which represented a more restrictive gun law environment, was associated with a 9% decrease in the odds of youth gun carrying (adjusted odds ratio [AOR], 0.91 [95% CI, 0.86-0.96]). Adult gun ownership mediated the association between state gun law score and youth gun carrying (AOR, 0.94 [ 95% CI, 0.86-1.01], with 29% attenuation of the regression coefficient from −0.09 to −0.07 based on bootstrap resampling). CONCLUSIONS AND RELEVANCE More restrictive overall gun control...