  Recently, it was suggested that large concentrations or groups of quasars may
trace sites of enhanced matter density at medium and high redshifts analogous
to how galaxy clusters trace them in nearby space. We have checked existing
quasar data for the presence of such groups. Large Quasar Groups (LQGs) were
identified using a well-known cluster analysis technique. Our final list
consists of 12 groups, including one reported previously. Most of the quasars
in these groups come from deep homogeneous surveys. We have found that: (i) the
probability that the detected groups are random is rather small (generally a
few per cent); (ii) their sizes range from $\sim 70$ to $\sim 160h^{-1} Mpc$,
which is comparable to sizes of nearby rich superclusters; (iii) the detected
groups all have redshifts $0.5<z\leq 2$; (iv) the abundance of the LQGs is
comparable with the abundance of large superclusters at $z\sim 0$, which is
consistent with the idea that quasar groups and superclusters can be
evolutionarily related. We argue that quasar groups could be a common feature
of the spatial distribution of medium redshift quasars, and that the quasars in
groups may belong to concentrations of young galaxy clusters and groups
(distant superclusters) and hence be biased tracers of large-scale structure of
matter distribution in the early Universe. Theoretical implications as well as
other observations needed to test this point are discussed.
