  A thorough analysis of well studied giant HII regions on galactic discs for
which we know the ionizing stellar population, the gas metallicity and the
Wolf-Rayet population, leads to photoionization models which can only match all
observed line intensity ratios ([OIII], [OII], [NII], [SII] and [SIII] with
respect to the intensity of H$\beta$), as well as the H$\beta$ luminosity and
equivalent width if one allows for an important escape of energetic ionizing
radiation. For the three regions presented here, the fractions of escaping
Lyman continuum photons amount to 10 to 73 % and, in all cases, the larger
fraction of escaping photons has energies between 13.6 and 24.4 eV. These
escaping photons clearly must have an important impact as a source of
ionization of the diffuse ionized gas (DIG) found surrounding many galaxies, as
well as of the intergalactic medium (IGM).
