Values of sea surface mean square slope (mss) computed from the backscattered power falloff with incidence angle measured by an airborne 36-GHz scanning radar altimeter are presented. When the mss values are plotted against the 10-m wind speed (extrapolated from the 46- to 110-m aircraft height), for wind speeds up to 7 m s-1 the values scatter about a curve which is scaled from that obtained by Wu <1990> by a factor of 0.85. Wu's result was a reanalysis of the optical measurements of Cox and Munk <1954>. The offset between the microwave and optical curves is reasonable because one would expect the surface to appear smoother to the microwave wavelength than to the optical. This is unlike the mss values inferred from absolute power measurements at nadir by 14-GHz satellite altimeters, which indicate a higher mss under light wind conditions than the optical measurements indicate. For wind speeds below 7 m s-1 the peak of the backscattered power distribution is observed at nadir, but for higher speeds the peak shifts off-nadir in the downwind direction. ¿ 1998 American Geophysical Union