  While all but one Gamma-Ray Bursts observed in the X-ray band showed an X-ray
afterglow, about 60 per cent of them have not been detected in the optical
band. We show that this is not due to adverse observing conditions. We then
investigate the hypothesis that the failure of detecting the optical afterglow
is due to absorption at the source location. We find that this is a marginally
viable interpretation, but only if the X-ray burst and afterglow emission and
the possible optical/UV flash do not destroy the dust responsible for
absorption in the optical band. If dust is efficiently destroyed, we are led to
conclude that bursts with no detected optical afterglow are intrinsically
different.
