  Recently, a new method to constrain the distances of blazars with unknown
redshift using combined observations in the GeV and TeV regimes has been
developed. The underlying assumption is that the Very High Energy (VHE, E > 100
GeV) spectrum corrected for the absorption of TeV photons by the Extragalactic
Background Light (EBL) via photon-photon interaction should still be softer
than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are
related to the real redshifts by a simple linear relation, that has been used
to infer the unknown or uncertain distance of blazars. The sample is revised
with the up-to-date spectra in both TeV and GeV bands and the method applied to
the unknown distance blazar PKS 1424+240 detected at VHE.
