  We report on ESO Very Large Telescope optical spectroscopy of 42 BL Lacertae
objects of unknown redshift. Nuclear emission lines were observed in 12
objects, while for another six we detected absorption features due to their
host galaxy. The new high S/N spectra therefore allow us to measure the
redshift of 18 sources. Five of the observed objects were reclassified either
as stars or quasars, and one is of uncertain nature. For the remaining 18 the
optical spectra appear without intrinsic features in spite of our ability to
measure rather faint (EW nearly 0.1 Angstrom) spectral lines. For the latter
sources a lower limit to the redshift was set exploiting the very fact that the
absorption lines of the host galaxy are undetected on the observed spectra.
