We have previously shown that breast-fed infants have a considerably larger thymus at 4 months than formula-fed infants. The aim of the present study was to investigate whether breast-feeding also influences the thymic size in late infancy. In a cohort of 50 infants, all being partially breast-fed when recruited at 8 months, ultrasound assessment of the thymic index (a volume estimate) was performed at both 8 and 10 months of age. At 10 months the thymic index was significantly higher in those still being breast-fed compared to infants who had stopped breast-feeding between 8 and 10 months of age (P=0.05). This difference became more significant when controlled for the influence of infectious diseases (P=0.03). In infants still breast-fed at 10 months there was a significant correlation between the number of breast-feeds per day and their thymic index (P=0.01). Conclusion The effect of breast-feeding on thymus size is likely to be caused by immune modulating factors in breast milk. Breast milk influences thymic size in late infancy.