  We present the results on the cosmological evolution of BL Lac objects as
derived from a statistically complete sample of 44 BL Lacs selected from the
X-ray bright tail of the REX survey. With this sample, we have investigated the
cosmological properties of BL Lacs taking into account the radio, optical and
X-ray limits. We infer that no evolution is clearly visible down to the flux
limits reached by our sample. On the other hand, deeper samples are probably
needed in order to detect the negative evolution found in the EMSS sample. The
identification of such deeper sample, extracted from the REX survey, is in
progress.
