  We present a theoretical model which aims at predicting the clustering
properties of X-ray clusters in flux-limited surveys for different cosmological
scenarios. The model uses the theoretical and empirical relations between mass,
temperature and X-ray luminosity, and fully accounts for the redshift evolution
of the underlying dark matter clustering and cluster bias factor. We apply the
model to the RASS1 Bright Sample and to the XBACs catalogue. The results show
that the Einstein-de Sitter models display too low a correlation length, while
models with a matter density parameter Omega_0m=0.3 (with or without a
cosmological constant) are successful in reproducing the observed clustering.
