In its recently adopted Global Strategy on Infant and Young Child Feeding, the World Health Assembly called for urgent action in addressing the barriers to optimal feeding practices. This paper examines mothers’ concerns about milk insufficiency as a major contributor to suboptimal infant feeding decisions, using survey data from periurban areas of two Bolivian cities. Mothers in the lowland modernizing city of Santa Cruz were more likely than mothers in the highland traditional city of Cochabamba to express concern about insufficient milk, and also less likely to feed their infants according to international recommendations. Furthermore, perceived milk insufficiency was particularly common among mothers of infants younger than 6 months of age – an age at which infants are particularly vulnerable to the adverse effects of breast-feeding cessation and complementary feeding initiation. The paper concludes with policy, programme and research recommendations to address the critical problem of perceived insufficient breast milk.