  A new method to constrain the distance of blazars with unknown redshift using
combined observations in the GeV and TeV regimes will be presented. The
underlying assumption is that the Very High Energy (VHE) spectrum corrected for
the absorption of TeV photons by the Extragalactic Background Light (EBL) via
photon-photon interaction should still be softer than the extrapolation of the
gamma-ray spectrum observed by Fermi/LAT. Starting from the observed spectral
data at VHE, the EBL-corrected spectra are derived as a function of the
redshift z and fitted with power laws. Comparing the redshift dependent VHE
slopes with the power law fits to the LAT data an upper limit to the source
redshift can be derived. The method is applied to all TeV blazars detected by
LAT with known distance and an empirical law describing the relation between
the upper limits and the true redshifts is derived. This law can be used to
estimate the distance of unknown redshift blazars: as an example, the distance
of PKS 1424+240 is inferred.
