The carry-over of fumonisin B1 from contaminated feed into dairy milk also suggests its carry-over from contaminated food into breast milk. This study assessed fumonisin B1 contamination in breast milk and associated exposures of infants under 6 months of age. Breast milk samples were collected from 131 lactating mothers and the weight of their infants was measured during the first month of lactation. Fumonisin B1 was extracted using methanol:acetone, cleaned up with Strong Anion Exchange columns and quantified by HPLC. Fumonisin B1 exposure in each child was estimated using deterministic approach. Out of the 131 samples, 58 (44.3%) contained fumonisin B1 at levels ranging from 6.57 to 471.05 ng/ml. Of the contaminated samples, 10.3% had fumonisin B1 levels above the EU limit of 200 ppb for fumonisins in infants' food. Exposure in the infants ranged from 0.78 to 64.93 µg/kg body weight (bw) per day (median, 3 µg/kg bw/day) and exceeded the provisional maximum tolerable limit of 2 µg/kg bw/day in 29% of the infants. In conclusion, breast milk from mothers in Northern Tanzania is contaminated with fumonisins at levels that lead to unacceptable exposures in infants. Strategies to prevent lactating mothers from fumonisin exposure are urgently needed to minimise fumonisin exposure in infants.