The paper gives a simple theoretical approach to the problem of cross-modulation distortion arising from frequency pulling in, or in direct conjunction with, a frequency-modulated klystron. An expression for the distortion spectrum relative to the fundamental component is derived, and on the basis of this expression a family of curves illustrating the severity of the distortion is computed and plotted on a logarithmic scale. It is shown that even small mismatches on the far end of a relatively short feeder into which the klystron is working give rise to serious crosstalk which, if not remedied, sets a limit to the quality of the modulator unit.