  We describe the initial results of a programme to detect and identify
extended X-ray sources found serendipitously in XMM-Newton observations. We
have analysed 186 EPIC-PN images at high galactic latitude with a limiting flux
of $1\times 10^{-14}$ \ergcms and found 62 cluster candidates. Thanks to the
enhanced sensitivity of the XMM-Newton telescopes, the new clusters found in
this pilot study are on the average fainter, more compact, and more distant
than those found in previous X-ray surveys. At our survey limit the surface
density of clusters is about 5 deg$^{-2}$. We also present the first results of
an optical follow-up programme aiming at the redshift measurement of a large
sample of clusters. The results of this pilot study give a first glimpse on the
potential of serendipitous cluster science with XMM-Newton based on real data.
The largest, yet to be fulfilled promise is the identification of a large
number of high-redshift clusters for cosmological studies up to $z=1$ or 1.5.
