Context : We present an estimation of cosmological parameters with clusters of galaxies .

Aims : We constrain the \Omega _ { m } , \sigma _ { 8 } , and w parameters from a stand-alone sample of X-ray clusters detected in the 50 deg ^ { 2 } XMM-XXL survey with a well-defined selection function .

Methods : We analyse the redshift distribution of a sample comprising 178 high S/N clusters out to a redshift of unity .
The cluster sample scaling relations are determined in a self-consistent manner .

Results : In a lambda cold dark matter ( \Lambda CDM ) model , the cosmology favoured by the XXL clusters compares well with results derived from the Planck S-Z clusters for a totally different sample ( mass/redshift range , selection biases , and scaling relations ) .
However , with this preliminary sample and current mass calibration uncertainty , we find no inconsistency with the Planck CMB cosmology .
If we relax the w parameter , the Planck CMB uncertainties increase by a factor of \sim 10 and become comparable with those from XXL clusters .
Combining the two probes allows us to put constraints on \Omega _ { m } = 0.316 \pm 0.060 , \sigma _ { 8 } = 0.814 \pm 0.054 , and w = -1.02 \pm 0.20 .

Conclusions : This first self-consistent cosmological analysis of a sample of serendipitous XMM clusters already provides interesting insights into the constraining power of the XXL survey .
Subsequent analysis will use a larger sample extending to lower confidence detections and include additional observable information , potentially improving posterior uncertainties by roughly a factor of 3 .