  We place tight constraints on the main cosmological parameters of spatially
flat cosmological models by using the recent angular clustering results of
XMM-{\it Newton} soft (0.5-2 keV) X-ray sources (Ebrero et al. 2009a), which
have a redshift distribution with a median of $z\sim 1$. Performing a standard
likelihood procedure, assuming a constant in comoving coordinates AGN
clustering evolution, the AGN bias evolution model of Basilakos et al. (2008)
and the WMAP5 value of $\sigma_8$, we find stringent simultaneous constraints
in the ($\Omega_{m}, $w) plane, with $\Omega_{m}= 0.26\pm 0.05$,
w$=-0.93^{+0.11}_{-0.19}$.
