Brief interventions effectively reduce alcohol problems; however, it is controversial whether longer interventions result in greater improvement. This study aims to determine whether an increase in treatment for people with more severe problems resulted in better outcome. We employed regression-discontinuity analyses to determine if drinking driver clients (n = 22,277) in Ontario benefited when they were assigned to a longer treatment program (8-hour versus 16-hour) based on assessed addiction severity criteria. Assignment to the longer16-hour program was based on two addiction severity measures derived from the Research Institute on Addictions Self-inventory (RIASI) (meeting criteria for assignment based on either the total RIASI score or the score on the recidivism subscale). The main outcome measure was self-reported number of days of alcohol use during the 90 days preceding the six month follow-up interview. We found significant reductions of one or two self-reported drinking days at the point of assignment, depending on the severity criterion used. These data suggest that more intensive treatment for alcohol problems may improve results for individuals with more severe problems. Copyright © 2012 John Wiley & Sons, Ltd.