This grounded theory study aimed to explore the cultural practices and behaviours, which lead to unsuccessful exclusive breastfeeding in Indonesian mothers. Data were obtained through semi-structured interviews carried out with 8 key participants (breastfeeding mothers) and 10 general participants (family members and health cadre). Four categories: the initial breastfeeding practice following delivery, the decision making surrounding the efforts to maintain breastfeeding, when to wean the baby onto other types of milk or food, and the consequences of weaning the baby, and two corresponding subcategories: external and internal factors that influence the decision making to wean the baby emerged from comparative analysis. Results from this study confirmed that women believed that they produced insufficient breast milk, which led them to wean their babies before the reccomended age of 6 months. This belief arose from a complex process and was strongly affected by various biopsychosocial factors. The results of this study will be useful in designing culturally sensitive nursing care and health programmes to support optimal breastfeeding for women of Indonesian origin.