Objective To determine whether motivational interviewing—a behavioural therapy for addictions—provided at home by specially trained midwives helps pregnant smokers to quit. Design Randomised controlled non-blinded trial analysed by intention to treat. Setting Clinics attached to two maternity hospitals in Glasgow. Participants 762/1684 pregnant women who were regular smokers at antenatal booking: 351 in intervention group and 411 in control group. Interventions All women received standard health promotion information. Women in the intervention group were offered motivational interviewing at home. All interviews were recorded. Main outcome measures Self reported smoking cessation verified by plasma or salivary cotinine concentration. Results 17/351 (4.8%) women in the intervention group stopped smoking (according to self report and serum cotinine concentration < 13.7 ng/ml) compared with 19/411(4.6%) in the control group. Fifteen (4.2%) women in the intervention group cut down (self report and cotinine concentration less than half that at booking) compared with 26 (6.3%) in the control group. Fewer women in the intervention group reported smoking more (18 (5.1%) v 44 (10.7%); relative risk 0.48, 95% confidence interval 0.28 to 0.81). Birth weight did not differ significantly (mean 3078 g v 3048 g). Conclusion Good quality motivational interviewing did not significantly increase smoking cessation among pregnant women.