A case-control study was conducted to investigate the dietary and socio-economic factors associated with beriberi in infants attending three public hospitals in Vientiane, Lao PDR. Forty-three breast-feeding infants with a median (range) age of 3 (1-9) months were admitted with beriberi. This was defined as the presence of signs of congestive heart failure or shock in the absence of fever or other signs of sepsis, hypovolaemia or cardiac abnormalities, with rapid clinical improvement following parenteral thiamine. Subjects were matched by age and diet to 43 breast-feeding healthy control infants. Compared with control mothers, mothers of infants with beriberi had significantly less diet diversity (p < 0.001), soaked glutinous rice for significantly longer or were more likely to pour off excess water from non-glutinous rice (p = 0.006), had fewer years of schooling (p < 0.05), were more likely to report that income was inadequate for basic needs (p < 0.001), to perform hard physical labour (p < 0.01) and to be married to farmers (p < 0.01). Clinically significant thiamine deficiency in breastfed infants in Lao relates to methods of preparing rice, the food selected by lactating mothers and the family's socio-economic status.