Observations of the Doppler spectral width made using the VHF profiler radar at White Sands Missile Range, New Mexico, are used to estimate the eddy diffusion coefficient Kz at 5--20 km on an hourly basis. The data used span the period 1991--1995. Medians of the hourly values of Kz range from ~3 m2/s near 5 km to about 0.3 m2/s near 14 km. The logarithms of the individual hourly values are approximately normally distributed. Seasonal median values of log Kz are nearly the same at all heights during autumn through spring, while the summer values are about twice as large as the values for other seasons. Hourly values of log Kz are negatively correlated with the static stability, are positively correlated with the vertical shear of the horizontal wind, and show weak negative correlation with the Richardson number during all seasons. The correlation of log Kz with the horizontal wind speed is positive in the winter, indicating that the magnitude of Kz is determined by synoptic weather systems then, and negative in the summer, indicating that convective processes are most important in determining Kz in the summer. The autocorrelation of log Kz as a function of vertical separation decreases more slowly with increasing vertical separation in the troposphere than in the stratosphere. The median values of log Kz that we find compare very well with those of past studies near the tropopause level and above, but in the troposphere they are significantly larger than those reported from the middle and upper atmosphere radar in Japan; this difference is discussed in terms of differences in the local terrain and the climate of the troposphere. ¿ 1997 American Geophysical Union