The efficiency of converting focused laser radiation into kilovolt x rays by means of a high‐zplanar plasma has been calculated. For electron temperatures of several keV, the rapid expansion of the plasma limits the effective absorption and reradiation times to a few tenths of a nanosecond. This short time limits the fractional degree of ionization to 30%–60%, depending on the temperature, for heavy target ions such as gold, and in this case the predominant ions have ionization energies less than the electron temperature. For plasma temperatures of 3 up to 10 keV the total radiation output will range from 6 down to 2 times that of plasma bremsstrahlung alone. The conversion efficiency is relatively insensitive to the laser frequency and to the plasma temperature, and amounts to 2%–5%. Overall efficiency for a Nd‐glass laser system turns out to be about 10−3.