This study characterized distribution patterns of monomethyl mercury (MeHg) and areal mass of total mercury (THg) and MeHg across U.S. upland forests. MeHg concentrations increased from surface litter (average: 0.14 μg kg(-1)) to intermediate (0.47 μg kg(-1)) and deeper, decomposed litter (1.43 μg kg(-1)). MeHg concentrations were lower in soils (0.10 μg kg(-1) at 0-20 cm depth; 0.06 μg kg(-1) at >20 cm depth). Ratios of MeHg to THg were higher in litter compared to soils. In soils, MeHg concentrations positively correlated with THg across all sites, and MeHg concentrations also increased with C content and latitude. THg areal mass ranged from 41.6 g ha(-1) to 268.8 g ha(-1). Largest THg mass at all sites was sequestered in soils (average of 91%), followed by litter (8%) and aboveground biomass (<1%). MeHg mass (litter plus soils only) ranged from 75 to 443 mg ha(-1), of which 88% was found in soils. Both THg and MeHg mass correlated with latitude, with average mass increases of 10.6 g ha(-1) (THg) and 20 μg ha(-1) (MeHg) per degree latitude, indicating that highest THg and MeHg accumulation in upland forests are expected in northern sites.