We examined gang interventions that produced adverse effects, or worse outcomes for treated participants in controlled evaluations. Prior studies describe harmful psychological processes and flawed research methods that can cause adverse effects in gang and crime prevention programs. We tested whether several program elements and methodological issues predicted adverse effects across 41 controlled evaluations. Only the presence of law enforcement officers as intervention providers significantly predicted adverse outcomes, and it is unclear whether law enforcement officers increased participants’ antisocial behaviors or simply detected more offending among treated participants. Including law enforcement officers in gang interventions may increase the risk of adverse outcomes; however, further research is needed to determine why.