This paper presents the results of a theoretical study of the sensitivity of Pn attenuation to the elastic and anelastic structure of the uppermost mantle for frequencies between 1 and 15 hz and distances between 200 and 1000 km. Synthetic seismograms are computed using wavenumber integration for an elastic model consisting of a crustal layer over an upper mantle with a slight linear velocity gradient (approximately equal to the earth-flattening transformation of a spherically symmetric homogeneous upper mantle). These are compared to synthetic seismograms for a crustal layer over a mantle halfspace (pure elastic head wave). We find that earth sphericity alone causes a significant departure in Pn attenuation from that of a canonical head wave (almost two orders of magnitude at 1000 km and 15 Hz), and that high frequencies attenuate less rapidly than low frequencies if the spherical earth velocity gradient is greater than or equal to zero. This implies that methods that assume frequency-independent Pn geometric spreading will produce biased estimates of the anelasticity of the upper mantle. This is demonstrated using synthetic seismograms computed for an anelastic spherical earth model. These results show that the velocity structure must be carefully considered when relating observed Pn spectral amplitudes to the anelastic structure of the upper mantle. ¿ American Geophysical Union 1990