Aims. To assess the impact of a brief intervention on antepartum alcohol consumption. Design. A randomized clinical trial. Setting. The obstetrics practices of the Brigham and Women's Hospital in Boston, MA, USA. Participants. Two hundred and fifty eligible women initiating prenatal care. Intervention. A comprehensive assessment of alcohol use (assessment only, AO) or the same comprehensive assessment with a brief intervention (BI). Measurement. Demographic background and obstetric history of subjects, current and lifetime use of alcohol and substances, composite Addiction Severity Index scores, and antepartum alcohol use. Findings. Of the 250, 247 (99%) subjects provided information on their antepartum drinking. Both the AO and BI groups had reductions in antepartum alcohol consumption, but differences in reductions by group were not statistically significant ( p > 0.05).Risk of antepartum drinking after either the AO or BI was increased nearly threefold if the subject had any prenatal alcohol consumption before assessment ( p = 0.0001). For the 143 subjects who were abstinent pre-assessment, however, those who received the BI maintained higher rates of abstinence (86% versus 72%, p = 0.04). Conclusions. After a comprehensive assessment of alcohol use, subjects in both the AO and BI groups reduced their antepartum alcohol consumption. The importance of screening for prenatal alcohol use is underscored by the findings that any prenatal alcohol consumption increases the risk of continued antepartum drinking.