Arsenic crosses the placenta and may have adverse consequences in utero and later in life. At present, little is known about arsenic concentrations in placenta and their relation to maternal and infant exposures particularly at common levels of exposure. We measured placenta arsenic in a US cohort potentially exposed via drinking water from private wells, and evaluated the relationships between placenta and maternal and infant biomarker arsenic concentrations. We measured total arsenic concentrations in placental samples from women enrolled in the New Hampshire Birth Cohort Study (N=766). We compared these data to maternal urinary arsenic (total arsenic and individual species) collected at approximately 24-28 week gestation, along with maternal post-partum toenails and infant toenails using non-parametric multivariate analysis of log10-transformed data. We also examined the association between placental arsenic and household drinking water arsenic. Placenta arsenic concentrations were related to arsenic concentrations in maternal urine (β 0.55, P value <0.0001), maternal (β 0.30, P value 0.0196) and infant toenails (β 0.40, P value 0.0293) and household drinking water (β 0.09, P value <0.0001). Thus, our data suggest that placenta arsenic concentrations reflect both maternal and infant exposures.