The evaluation used a systematic approach to track 775 driving under the influence (DUI) offenders sentenced to use interlock as a condition of probation in 4 California counties: Alameda; San Diego; Sonoma; and Santa Clara. Conclusions concerning the success of the interlock in reducing recidivism are based on a comparison between 584 DUI probationers who had the devices installed in their cars with a group of 506 matched DUI offenders who were not sentenced to use the devices. The findings on recidivism and on implementation support of a positive but non-conclusive assessment of the potential of ignition interlock as a DUI countermeasure. Further supportive policy within certain limits is recommended for developing the potential of ignition interlock as a comprehensive approach to reducing drinking and driving problems. The recommendations deal with areas of communication of information concerning interlock sentence, installation, and compliance to all necessary agencies, sentencing decisions, sanctions, implementation responsibilities, and cost considerations.