  The Large Hadron Collider (LHC) of the European Organisation for Nuclear
Research (CERN) started its operation in Autumn of 2009. The initial run at a
centre-of-mass energy of 900 GeV, has been followed by the on-going run at the
energy of 7 TeV. While initially the probability of several proton-proton
collisions to happen within the same bunch-crossing was approximately 10^-5,
the level of the pile-up grows steadily with better focusing and squeezing of
the LHC beams at collision point. Presented in this contribution is the
performance of the primary vertex reconstruction algorithms used for analysis
of the first collisions at the LHC. Different approaches used for the
reconstruction of primary vertices in 900 GeV and 7 TeV collisions are
presented. The efficiencies of the primary vertex reconstruction used for the
first physics analyses of ATLAS are shown. The resolutions on positions of the
reconstructed primary vertices are investigated by studying the distributions
of pulls of distance between artificially created half-vertices. Implications
of the ATLAS performancewith respect to primary vertex reconstruction for the
on-going and future physics analyses are discussed.
