Background: Smoking in the postpartum period may contribute to early weaning, although the nature and temporal aspect of the relationship are poorly understood. The objective of this study was to examine the association between early weaning and smoking relapse among women who stopped smoking during pregnancy. Methods: A secondary analysis of data from a randomized controlled trial was conducted. The participants were 228 women who had stopped smoking for pregnancy, who participated in a smoking relapse prevention trial, and who breastfed. Women who relapsed to daily smoking postpartum were compared with those who remained abstinent or smoked occasionally. The dependent variable was breastfeeding for less than 26 weeks (early weaning). Potential covariates included intended duration of breastfeeding, parity, partner's smoking, nicotine dependence, emotional health, return to paid employment, and various sociodemographic variables. Results: Approximately two-thirds (65.1%) of the women who relapsed to daily smoking weaned before 26 weeks compared with 33.8 percent of the women who remained abstinent or smoked occasionally. Controlling for intended duration of breastfeeding, education, and return to paid employment, women who resumed daily smoking were almost four times more likely to wean early than those who abstained or smoked occasionally. Conclusions: Early weaning may result from psychological or physiological changes associated with tobacco use. Smoking relapse prevention in the postpartum period may be one of the most effective interventions in ensuring that women who stop smoking for pregnancy remain stopped and breastfeed their babies for the recommended duration.