Objectives. To determine differences among US states in how driving under the influence of alcohol (DUI) laws activate federal firearm possession and purchase prohibitions.Methods. We performed primary legislative research to characterize DUI laws in each state. The primary outcome was the number of DUI convictions an individual must be convicted of in each state to activate the federal firearm possession and purchase prohibition. We also determined the time interval in which previous DUI convictions count for future proceedings.Results. Forty-seven states had DUI laws that activated the federal prohibition of firearm possession and purchase for a threshold number of repeated DUIs. Variation exists among states in the number of convictions (1-4) and length of liability period (5 years-lifetime) required to prohibit firearm possession and purchase.Conclusions. Variation in state laws on DUI results in differences in determining who is federally prohibited from possessing and purchasing firearms. Future research should explore whether these federal prohibitions arising from DUI convictions are enforced and whether an association exists between stricter DUI policies and reduction in firearm crimes, injuries, and deaths.