Q values for Lg and its coda exhibit significant lateral variations across the United States, some of which correlate with known variations in the thickness of shallow sedimentary layers. The possibility that all variations in Lg attenuation are caused by changes in the thickness of shallow sediments is investigated by computing multi-mode synthetic seismograms for crustal models with various thicknesses of low-Q sediments. Using crustal velocity and Q models obtained in previous studies, and reasonable values for the thicknesses and intrinsic Q values of sediments in several regions, we find that the variation of Lg Q values between the Appalachian and Rocky Mountains can easily be produced by varying accumulations of sandstone and shale of Mesozoic age and younger. By contrast, throughout most of the Western United States, neither the low Lg Q values which have been observed nor the regional variation of those values can be explained by accumulations of low-Q sediments. Instead, their explanation appears to require low and laterally varying values of Q in the crystalline crust.