This study investigated, under real-world conditions, whether a statewide 2-year administrative ignition interlock license restriction program in Maryland was effective in reducing subsequent alcohol-related traffic violations among multiple offenders and whether any reductions in recidivism could be maintained after the program ended and interlock license restrictions were removed. A total of 1,927 drivers eligible for relicensure were randomly assigned to either the 2-year interlock license restriction program or the normal and customary sanctions afforded multiple offenders in Maryland. Recidivism was defined as incurring a subsequent alcohol-impaired driving violation during the 2-year intervention or 2-year postintervention periods. Compared to the control group, participation in the interlock license restriction program reduced drivers' hazard (or risk) of a subsequent alcohol-impaired driving offense by a statistically significant 36 percent during the 2-year intervention, 26 percent during the 2-year postintervention period, and 32 percent during the entire 4-year study period. This investigation of interlock program effectiveness is the first to report significantly lower recidivism among the interlock group than its control group after the ignition interlock license restriction program ended. Possible reasons for this novel finding and areas for future research are discussed. (Published Abstract)