Synthetic musk compounds are used as additives in many consumer products, including perfumes, deodorants, and detergents. Earlier studies have reported the occurrence of synthetic musks in environmental and wildlife samples collected in the United States. In this study, human breast milk samples collected from Massachusetts, were analyzed for the determination of concentrations of synthetic musks such as musk xylene (1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene), musk ketone (4-tert-butyl-2,6-dimethyl-3,5-dinitroacetophenone), HHCB (1,3,4,6,7,8-hexahydro-4,6,6,7,8,8-hexamethylcyclopenta[gamma]-2-benzopyran), AHTN (7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydronaphthalene), and HHCB-lactone, the oxidation product of HHCB. In addition, we estimated the daily intake of synthetic musks by infants based on the ingestion rate of breast milk. Synthetic musks were found in most of the samples analyzed, and the concentrations ranged from < 2 to 150 ng musk xylene/g, < 2 to 238 ng musk ketone/ g, < 5 to 917 ng HHCB/g, < 5 to 144 ng AHTN/g, and < 10 to 88.0 ng HHCB-lactone/g, on a lipid weight basis. The concentrations of HHCB were higher than the concentrations of other synthetic musks in breast milk samples. The mean concentration of HHCB (220 ng/g, lipid weight) was 5 times greater than the concentrations reported 10 years ago for breast milk samples collected in Germany and Denmark. Maternal age was not correlated with the concentrations of musk xylene, musk ketone, HHCB, or AHTN. There was a trend of decreasing concentrations of musk xylene, musk ketone, HHCB, and AHTN, with the number of children previously breast-fed, although the correlation was not significant. Based on average daily ingestion rate of breast milk, an infant is estimated to ingest 297 +/- 229 ng musk xylene, 780 +/- 805 ng musk ketone, 1830 +/- 1170 ng HHCB, 565 +/- 614 ng AHTN, and 649 +/- 598 ng HHCB-lactone per day. The ingestion rate of synthetic musks by infants in the United States is lower than that estimated for persistent organic pollutants (POPs) such as polychlorinated biphenyls (PCBs). Based on the residue patterns and accumulation features, it can be concluded that the exposure characteristics for synthetic musks are different from those of POPs, and that the major source of exposure to synthetic musks is probably via dermal absorption or inhalation.