BACKGROUND Breastfed infants rely on adequate maternal dietary iodine intake. OBJECTIVE Our objective was to measure breast milk iodine and perchlorate, an inhibitor of iodide transport into the thyroid and potentially into breast milk, in Boston-area women. METHODS The study included 57 lactating healthy volunteers in the Boston area. METHODS Breast milk iodine and perchlorate concentrations and urine iodine, perchlorate, and cotinine concentrations were measured. For comparison, iodine and perchlorate levels in infant formulae were also measured. RESULTS Median breast milk iodine content in 57 samples was 155 microg/liter (range, 2.7-1968 microg/liter). Median urine iodine was 114 microg/liter (range, 25-920 microg/liter). Perchlorate was detectable in all 49 breast milk samples (range, 1.3-411 microg/liter), all 56 urine samples (range, 0.37-127 microg/liter), and all 17 infant formula samples (range, 0.22-4.1 microg/liter) measured. Breast milk iodine content was significantly correlated with urinary iodine per gram creatinine and urinary cotinine but was not significantly correlated with breast milk or urinary perchlorate. CONCLUSIONS Perchlorate exposure was not significantly correlated with breast milk iodine concentrations. Perchlorate was detectable in infant formula but at lower levels than in breast milk. Forty-seven percent of women sampled may have been providing breast milk with insufficient iodine to meet infants' requirements.