Magnetosheath data from IMP 8 and solar wind data from ISEE 1, ISEE 3, and WIND are used to investigate Earth's magnetosheath structure. Magnetosheath data are transformed into a coordinate system which allows data from regions with similar physical histories to be binned together. The results show a significant dawn-dusk asymmetry in Earth's magnetosheath near solar maximum but not near solar minimum, with larger densities on the dawnside than on the duskside. We use a magnetohydrodynamic (MHD) simulation to model the Parker spiral interplanetary magnetic field (IMF) case; an asymmetry does develop in the same sense as observed in the data but with a somewhat smaller magnitude. Near solar minimum the observed density asymmetry is dependent on the IMF orientation, but this is not true near solar maximum, apparently ruling out both foreshock effects and different compressions/deflections by parallel and perpendicular shocks as causes. Earth's magnetosphere thus could be a contributor to this effect. ¿ 2001 American Geophysical Union