  We examine the luminosity and dynamical mass estimates for young massive
stellar clusters. For many young (<50 Myr) clusters, the luminosity and
dynamical mass estimates differ by a significant amount. We explain this as
being due to many young clusters being out of virial equilibrium (which is
assumed in dynamical mass estimates) because the clusters are undergoing
violent relaxation after expelling gas not used in star formation. We show
that, if we assume that luminous mass estimates are correct (for a standard
IMF), at least 50 per cent of young clusters for which dynamical masses are
known are likely to be destroyed within a few 10s Myr of their formation. Even
clusters which will retain a bound core may lose a large fraction of their
stellar mass. We also show that the core radius and other structural parameters
change significantly during the violent relaxation that follows gas expulsion
and that they should be considered instantaneous values only, not necessarily
reflecting the final state of the cluster. In particular we note that the
increasing core radii observed in young LMC/SMC clusters can be well explained
as an effect of rapid gas loss.
