Rough surface scattering theory is applied to predict the directional asymmetry of microwave backscatter from the sea for assumed forms of the surface directional wave number spectrum. Several wave spectral models exist that describe the directional distribution of gravity waves near their spectral peak. These waves are shown to govern the directionality of microwave backscatter at low incidence angles for light winds. Under such conditions the scattering asymmetry predicted using these models exceeds that observed at microwave frequencies of 5.3 and 14 GHz. Effects of the spatial and temporal variability of directional seas and the proximity of land are insufficient to account for this difference. The assumption of similarity that is common to all of the gravity wave spectral models appears to be a more likely source of the disparity between measured and predicted scattering asymmetry. Directional wave data acquired using oceanographic buoys are found also to exhibit a wind speed dependence inconsistent with the similarity principle.