  We study clustering on very large scales -- from several tens to hundreds of
comoving Mpc -- using an extensive catalog of heavy-element QSO absorption-line
systems. We find significant evidence that C IV absorbers are clustered on
comoving scales of 100 $h^{-1}$ Mpc ($q_0=0.5$) and less. The superclustering
is present even at high redshift ($z\sim 3$); furthermore, it does not appear
that the superclustering scale (comoving) has changed significantly since then.
Our estimate of that scale increases to 240 $h^{-1}$ Mpc if $q_0=0.1$, which is
larger than the largest scales of clustering seen at the present epoch. This
may be indicative of a larger value of $q_0$, and hence $\Omega_0$. We identify
7 high-redshift supercluster candidates, with 2 at redshift $z\sim 2.8$. The
evolution of the correlation function on 50 $h^{-1}$ Mpc scales is consistent
with that expected in cosmologies with $\Omega_0= $ ranging from 0.1 to 1.
Finally, we find no evidence for clustering on scales greater than 100 $h^{-1}$
Mpc ($q_0=0.5$) or 240 $h^{-1}$ Mpc ($q_0=0.1$).
