  We present the first results from an XMM-Newton observation of RX
J1347.5-1145 (z=0.451), the most luminous X-ray cluster of galaxies currently
known, with a luminosity L_X = 6.0 \pm 0.1 \times 10^{45} erg/s in the [2-10]
keV energy band. The cluster has an overall temperature of kT=10.0 \pm 0.3 keV
and is not isothermal: the temperature profile shows a decline in the outer
regions and a drop in the centre, indicating the presence of a cooling core.
The spectral analysis identifies a hot region at radii 50-200 kpc to south-east
of the main X-ray peak, at a position consistent with the subclump seen in the
X-ray image. Excluding the data of the south-east quadrant, the cluster appears
relatively relaxed and we estimate a total mass within 1 Mpc of 1.0 \pm 0.2
\times 10^{15} M_sun.
