Perchlorate is most widely known as a solid oxidant for missile and rocket propulsion systems and has been detected in drinking water, fruits, vegetables, and milk throughout New Jersey and most of the U.S. Perchlorate interferes with the uptake of iodine into the thyroid and may interfere with the development of the skeletal system and central nervous system of infants who ingest perchlorate. Therefore, it is important to understand the occurrence of perchlorate in breast milk. This study will allow us to acquire valuable information on human exposure to perchlorate through analyses of breast milk, urine, and drinking water. One hundred and six lactating mothers were recruited from the Eric B. Chandler health center in New Brunswick, NJ and provided consent to participate in this study. Each subject was asked to provide three sets of samples and complete a 24-hour dietary recall. Regression analyses showed that diet was the greatest predictor of perchlorate in breast milk and urine and that drinking water was not a major source of perchlorate. The average perchlorate level in drinking water samples was 0.168 ng/mL (n = 253), which is below New Jersey's Maximum Contaminant Level of 5 ng/mL. Perchlorate was detected in all breast milk samples provided (n = 276). The average perchlorate level in breast milk was 6.80 ng/mL. These findings are consistent with widespread perchlorate exposure in lactating women and infants, and that breast milk is a viable exposure route for infants. Based on the measured perchlorate levels in breast milk, we estimate that 25% of infants 0-6 months of age would exceed the EPA's reference dose of 0.0007 mg/kg/day. Breast-feeding is important in an infant's growth and development. Therefore, it is important to gain a better understanding of environmental contaminant concentrations in human breast milk and other sources of infant nourishment.