Brief motivational interventions (BMIs) are usually effective for reducing alcohol use and consequences in primary care settings. We examined readiness to change drinking as a mediator of the effects of BMI on alcohol-related consequences. Participants were randomized into three conditions: (a) standard care plus assessment (SC), (b) SC plus BMI (BI), and (c) BI plus a booster session (BIB). At 12-month follow-up BIB patients had significantly reduced alcohol consequences more than had SC patients. Patients receiving BI or BIB maintained higher readiness scores 3 months after treatment than did patients receiving SC. However, readiness mediated treatment effects only for those highly motivated to change prior to the intervention but not for those with low pre-intervention motivation. BI and BIB for these patients decreased alcohol consequences in part because they enhanced and maintained readiness for those highly motivated prior to the intervention, but not for those with low motivation. Results are opposite of what would be expected from MI theory. An alternative explanation is offered as to why this finding occurred with this opportunistically recruited Emergency Department patient population.