To evaluate the effectiveness of a physician-based intervention to promote smoking cessation during pregnancy, we conducted this randomized controlled trial in the resident-staffed prenatal clinics at the University of North Carolina Women's Hospital . Two hundred fifty prenatal patients who smoked were enrolled at their first visit and randomly assigned to the intervention or the usual-care group. Resident physicians provided self-help materials to intervention subjects and used a script to set goals with them at each prenatal visit. Subjects who set quit dates were contacted by volunteer cessation counselors. To verify smoking status, subjects provided a self-report and breath carbon monoxide (CO) sample at each visit. Controls were similarly assessed at enrollment and at three additional predetermined intervals. Twenty percent of intervention subjects and 10% of controls reported cessation, which was verified by CO level (P = .052). Fifty-one percent of subjects reduced their consumption by half or more, compared with 30% of controls (P = .002). The intervention is effective in promoting smoking cessation and reduction. In addition, this technique is inexpensive, readily accepted by staff, and efficient.