Objective: To determine the effectiveness of an intensive quit-smoking intervention on smoking rates at 36 weeks' gestation among pregnant Aboriginal and Torres Strait Islander women. Design: Randomised controlled trial. Setting and participants: Pregnant Aboriginal and Torres Strait Islander women (n= 263) attending their first antenatal visit at one of three Aboriginal community-controlled health services between June 2005 and December 2009. Intervention: A general practitioner and other health care workers delivered tailored advice and support to quit smoking to women at their first antenatal visit, using evidence-based communication skills and engaging the woman's partner and other adults in supporting the quit attempts. Nicotine replacement therapy was offered after two failed attempts to quit. The control ("usual care") group received advice to quit smoking and further support and advice by the GP at scheduled antenatal visits.Main outcome measure: Self-reported smoking status (validated with a urine cotinine measurement) between 36 weeks' gestation and delivery. Results: Participants in the intervention group (n = 148) and usual care group (n= 115) were similar in baseline characteristics, except that there were more women who had recently quit smoking in the intervention group than the control group. At 36 weeks, there was no significant difference between smoking rates in the intervention group (89%) and the usual care group (95%) (risk ratio for smoking in the intervention group relative to usual care group, 0.93 [95% CI, 0.86-1.08]; P = 0.212). Smoking rates in the two groups remained similar when baseline recent quitters were excluded from the analysis. Conclusion: An intensive quit-smoking intervention was no more effective than usual care in assisting pregnant Aboriginal and Torres Strait Islander women to quit smoking during pregnancy. Contamination of the intervention across groups, or the nature of the intervention itself, may have contributed to this result.