  We announce the detection of correlations on angular scales of $\ga10'$
between optically bright, high-redshift, radio-loud QSOs with diffuse X-ray
emission seen by ROSAT in the {\it All-Sky Survey}. These correlations reach
significance levels of up to $99.8\%$. A comparison of the results with a
sample of control fields, bootstrapping analyses, and Kolmogorov-Smirnov tests
provide unambiguous support for the statistical significance of the
correlations found. We argue that the detected enhanced diffuse X-ray emission
is in the foreground of the QSOs, and that it is probably due to galaxy
clusters which magnify the QSOs by their gravitational lensing effect, thereby
giving rise to a magnification bias in the background source sample. A
comparison of the results presented below with correlations previously found
between the same QSO sample and either Lick or IRAS galaxies provides further
evidence for this interpretation, and identifies positions in the sky where
weak gravitational lensing may be detected by searching for coherent
distortions of background galaxy images.
