Previous authors have associated large events, or ''sea spikes'', in the radar cross section of the sea surface with steep or breaking waves. Here we present a method based on an intensity threshold and the mean Doppler frequency to identify and quantify sea spikes likely to be associated with large-scale wave breaking. This method is applied to microwave measurements of Ku band made with a dual-polarized Doppler scatterometer at a moderate incidence angle (45¿) from a fixed platform in the North Sea. For VV and HH polarization the mean normalized radar cross section &sgr;0, the frequency of sea spike occurrence N, and the sea spike contribution to the mean normalized radar cross section, &sgr;SS0, are correlated with direct measurements of the friction velocity, u*, in the range 13≤u*≤49 cm s-1. The friction velocity exponents for both N and &sgr;SS0 indicate an approximately cubic dependence, while the residual radar cross section &sgr;res0=&sgr;0 -&sgr;SS is approximately proportional to friction velocity for u* greater than approximately 20 cm s-1. At high friction velocities, (u*≈40--50 cm s-1) these data show that the contribution of large sea spikes to the mean radar cross section for measurements between 25¿ and 45¿ relative to the wind is in the range of 5--10% for VV polarization and 10--20% for HH polarization. ¿ American Geophysical Union 1990