OBJECTIVE: To examine the association of breastfeeding duration with psychosocial development at 6 years of age. METHODS: We analyzed data from the 2005-2007 Infant Feeding Practices Study II and its 2012 Year 6 Follow-Up (N = 1442). Our breastfeeding duration variable combined overall and exclusive breastfeeding reported during infancy (never breastfed, breastfed <6 months, breastfed >/=6 months + exclusive breastfeeding <3 months, and breastfed >/=6 months + exclusive breastfeeding >/=3 months). Maternal responses to the Strengths and Difficulties Questionnaire were used to create our child psychosocial outcome domains (emotional symptoms, conduct problems, hyperactivity, peer problems, prosocial behavior, and total difficulties). Separate multivariable logistic regression models controlling for maternal sociodemographic characteristics, maternal mental health, and child characteristics were used to assess the likelihood of having difficulties on the 6 domains based on breastfeeding duration. RESULTS: Compared with children who were never breastfed, those who were breastfed for >/=6 months and exclusively breastfed for >/=3 months had decreased odds of difficulties with emotional symptoms (odds ratio [OR]: 0.52; 95% confidence interval [CI]: 0.27-0.99), conduct problems (OR: 0.24; 95% CI: 0.10-0.54), and total difficulties (OR: 0.39; 95% CI: 0.18-0.85) before adjustment. These associations were no longer significant after adjustment. CONCLUSIONS: Although in our unadjusted analyses we observed significant associations between breastfeeding duration and later psychosocial development, including decreased odds of emotional, conduct, and total difficulties at 6 years of age, these findings were no longer detectable after adjusting for the many potential confounding factors that play a role in psychosocial development.