Argentina and Uruguay are among the countries in which a large proportion of young women smoke. The rate of smokers during pregnancy in both countries is not well known, and data on the characteristics of women who quit smoking during pregnancy compared to those who continue smoking are not available. We conducted a survey including 1512 pregnant women > or = 18 years old (796 in Argentina; 716 in Uruguay), during antenatal visits in public hospitals of large urban regions; 44% of the women in Argentina and 53% in Uruguay had been or were regular smokers. 11% of the surveyed women in Argentina and 18% in Uruguay continued smoking during pregnancy. In both countries, the proportion of women who lived with smokers, allowed smoking at home, and were regularly or always exposed to tobacco smoke indoors, were 49%, 46% and 20% in the subgroup of women who never smoked, 67%, 60% and 32% in those who quit, and 78%, 75% and 52% in those who continued smoking, respectively. The study confirms a serious public health problem in both countries, and documents that environmental exposure persists in subgroups of women, even in those who quit smoking. It is important that the public health sector should provide access to effective programs for smoking cessation, to women who smoke during pregnancy. For the development of a new program, any intervention intending to have at least a moderate and sustainable success, it should seriously consider including components targeting the smoking environment of the pregnant women who smoke.