  We developed a numerical simulation of the propagation of UHECR in the Cosmic
Microwave Background (CMB) and we used it to determine the significance of the
GZK feature in the spectrum of UHECR measured by AGASA and HiRes. We find that
these two experiments are best fit by two different injection spectra in the
region below 10^20 eV and that the error bars around the GZK feature are
dominated by fluctuations which leave a determination of the GZK feature not
attainable at present. In addition the comparison of the spectra of AGASA and
HiRes suggests the presence of about a 30% systematic errors in the relative
energy determination of the two experiments. Correcting for these systematics,
the two experiments are brought in agreement at energies below 10^20 eV and, in
this region, are best fit by an injection spectrum with spectral index 2.5-2.6.
In the high energy region (above 10^20 eV) the two experiments maintain their
disagreement, but only at the 2sigma level. Our results clearly show the need
for much larger experiments such as Auger, EUSO, and OWL, that can increase the
number of detected events by one or two orders of magnitude.
