The protective effect of anti-Giardia antibodies in mother's milk on the acquisition of Giardia infection in their children during the first 2 y of life was analysed as part of a prospective study on infant diarrhoea in a group of 307 mothers and children in Leòn, Nicaragua. Among 24 children acquiring infection within the first 6 months, 23 were born to mothers lacking antibodies. These children also developed more severe diarrhoea. A significant difference between children born to mothers with and without antibodies with respect to the age at which the first Giardia infection was acquired was demonstrated by survival analysis and log rank test (p = 0.036). In conclusion, children born to non-immune mothers are at significantly higher risk of acquiring Giardia infection and developing giardiasis with more severe symptoms compared with children of immune mothers.