The self-focusing instability of high-power radio waves in the ionosphere in the presence of density irregularities is presented. The present study addresses the role of preexisting density irregularities which are always present in the instability region. The presence of ambient irregularities results in the excitation of wave numbers, which, on the basis of homogeneous theory of the self-focusing instability, should be stable. This effect can explain the puzzling observations that indicate growth of the medium-scale (on the order of hundreds of meters) irregularities during ionospheric heating. The computational results are compared with available experimental data and will be used to plan experiments using the upcoming High-Frequency Active Auroral Research Program heater. ¿ American Geophysical Union 1996