About 23% of U.S. women smoke at conception, and 54% of them report quitting spontaneously during pregnancy. However, large numbers of women who quit during pregnancy return to smoking (“relapse”) after delivery, e.g., 45% and 81% relapse by 6 and 12 months postpartum, respectively. Postpartum smoking relapse not only limits benefits of smoking abstinence to the mother, but also has serious negative consequences for child health and development through tobacco metabolites in breast milk and ambient air. There is little success in preventing long-term (9 months or longer) postpartum smoking relapse by existing interventions. Breastfeeding mothers tend to have lower risk of postpartum smoking relapse than formula-feeding mothers, although the causality of this association is unclear. It is possible that ex-smoking mothers who are less likely to relapse choose to breastfeed longer. But the reverse may also be true, i.e., mothers who breastfeed may be less likely to relapse, partly due to the hormonal (high oxytocin and prolactin), emotional, and mother-infant relationship benefits resulting from breastfeeding. A randomized controlled trial to reduce postpartum smoking relapse by promoting breastfeeding may not only move the field forward by addressing the uncertainty of causality, but also yield a new approach to reducing the large public health problem of postpartum smoking relapse that has long lasting benefits for mothers and infants. We propose the first randomized controlled trial to reduce postpartum smoking relapse by breastfeeding promotion from late pregnancy to 6 months postpartum. We will enroll 60 pregnant women who successfully quit smoking within 3 months before or during this pregnancy (<28 weeks) and randomize them into either the breastfeeding intervention group (N=30) or the control group (N=30). The intervention group will receive multicomponent interventions consisting of prenatal breastfeeding education, postnatal lactation counseling, family/peer/employer support, contingent financial incentives, and early limited formula milk via syringe (optional for at-risk infants). The control group will receive only a brief pamphlet on breastfeeding education. The primary outcome is timing of postpartum smoking relapse verified by urine cotinine test. Specific Aim 1 is to examine the efficacy of breastfeeding promotion intervention on postpartum smoking relapse. Specific Aim 2 is to collect preliminary data regarding potential mediating mechanisms: increases in lactation hormones (plasma oxytocin and prolactin), enhancing mother-infant relationship (bonding and positive maternal and infant behaviors during interactions), and reductions in stress and negative affect. This innovative and feasible pilot study can yield promising preliminary data to strongly support our future application of a NIH R01 grant proposal that has the potential to reduce smoking-related harms to both the mother and the infant, and maximize the protective effects of...