  We study the impact of modern stellar atmospheres that take into account the
effects of stellar winds, departures from LTE and line blanketing ("CoStar"
models) on the ionization structure of HII regions. Results from a large grid
of photoionization models are presented. Due to a flatter energy distribution
in the HeI continuum, compared to the widely used Kurucz models, generally
higher ionic ratios are obtained.
  We find that N+/O+ and Ne++/O++ can be safely used as direct indicators of
N/O and Ne/O abundance ratios in HII regions, over a wide range of
astrophysical situations.
  The roughly constant observed value of Ne++/O++ ionic ratios in Galactic HII
regions is naturally reproduced by photoionization models using CoStar fluxes,
while Kurucz models at solar metallicity fail to reproduce this behaviour. This
gives support to ionizing fluxes from non-LTE atmospheres including stellar
winds and line blanketing. However, we also point out that tests of stellar
atmosphere models from observations of HII regions are hampered by a lack of
strong constraints on the ionization parameter.
