Objective. To evaluate the efficacy of a brief smoking cessation intervention with pregnant women practicable routinely by midwives. Design. Midwives were randomized to deliver the experimental intervention or usual care. The 10‐15‐minute intervention was based on brief counselling, written materials, arrangements for continuing self‐help support and feedback on expired‐air carbon monoxide levels. The intervention was tailored to the women's needs: those who did not want to stop smoking received a brief motivational intervention, those who wanted to stop received an intervention designed to assist them and those that had stopped recently (recent ex‐smokers) received a relapse prevention intervention. Setting. Booking interviews with pregnant women in nine hospital and community trusts. Subjects. A total of 1120 pregnant women in the third month of pregnancy (249 recent ex‐smokers and 871 current smokers). Main outcome measures. Three indicators of biochemically validated abstinence were collected. Continuous abstinence for at least 3 months prior to delivery, point prevalence abstinence immediately post‐delivery, and continuous abstinence from 3 months pre‐delivery to 6 months post‐delivery. Results. Only a small proportion of the women who would have been eligible to take part in the trial were actually recruited by 178 recruiting midwives, with lack of time being cited as the main barrier. The intervention and usual care groups differed in post‐delivery point prevalence abstinence rates for recent ex‐smokers (65% vs. 53%, p < 0.05, one‐tailed), but not in other outcome measures. Overall, 54% of 'recent ex‐smokers' at booking and 7% of 'current smokers' at booking had been abstinent for at least 3 months at the time of delivery, and 23% and 3%, respectively, were still abstinent by the time the child was 6 months old (i.e. 12 months post‐intervention). Smoking status at follow‐up was predicted by dependence indexed by time to first cigarette in the morning . Conclusions. A brief 'one‐off' smoking cessation intervention by midwives does not seem to be a practicable or effective method of helping pregnant smokers to stop. Other options such as tailored self‐help materials and telephone counselling and other specialist treatments should be examined. Current smoking cessation rates in pregnancy are very low.