  We present the results of a search for clustering among the highest energy
events detected by the surface detector of the Pierre Auger Observatory between
1 January 2004 and 31 August 2007. We analyse the autocorrelation function, in
which the number of pairs with angular separation within a given angle is
compared with the expectation from an isotropic distribution. Performing a scan
in energy above 30 EeV and in angles smaller than 30 degrees, the most
significant excess of pairs appears for E > 57 EeV and for a wide range of
separation angles, between 9 and 22 degrees. An excess like this has a chance
probability of about 2% to arise from an isotropic distribution and appears at
the same energy threshold at which the Pierre Auger Observatory has reported a
correlation of the arrival directions of cosmic rays with nearby astrophysical
objects.
