  Variability is one of the hallmarks of Active Galactic Nuclei. The Burst
Alert Telescope onboard of Swift, with its homogeneous coverage of the sky is a
formidable tool to study variability at hard X-rays. We present here the
analysis of the 1-month binned Swift/BAT lightcurves of the 20 brightest Active
Galactic Nuclei in the hard X-ray sky. The sample consists of 2 blazars, 3
radio galaxies, 6 Seyfert 1/1.5s, 8 Seyfert 2s and 1 Narrow Line Seyfert 1. We
found that all the objects show variability, and most of them have a value of
the fractional root mean squared variability amplitude of Fvar \sim 0.2 - 0.3.
We did not find any significant correlation of Fvar with the column density or
the luminosity in our sample.
