BACKGROUND: Breastfeeding is increasingly being promoted worldwide. Although several studies have examined breastfeeding and postpartum depression, contradictory results concerning their relationship have been found. This study investigated the influence of the feeding patterns of 1- to 6-month-old infants on maternal postpartum depression, as well as the influence of activities performed by mothers during feeding on postpartum depression. METHODS: We used data from parents and children who participated in the Japan Environment and Children's Study (JECS). The data were from 71,448 mothers who did not show depressive symptoms at 1 month postpartum. RESULTS: The group that continued exclusive breastfeeding for 6 months postpartum had a lower risk of postpartum depression compared with those who adopted other feeding patterns. Regardless of the pattern or duration of feeding, the group that maintained eye contact or talked to their baby during feeding had a lower risk of postpartum depression compared with the group that performed other activities. Furthermore, the group that continued both exclusive breastfeeding for 6 months and maintained eye contact or talked to their babies during feeding had an odds ratio of 0.69 for postpartum depression (95% confidence interval: 0.61-0.79), the lowest of any group. LIMITATIONS: All variables were measured using a self-administered questionnaire. CONCLUSIONS: It may be possible to control the onset of postpartum depression by recommending breastfeeding to new mothers, providing them with appropriate information on how to interact with their babies, and offering them support. TRIAL REGISTRATION: UMIN000030786.