  We prove that extreme Kerr initial data set is a unique absolute minimum of
the total mass in a (physically relevant) class of vacuum, maximal,
asymptotically flat, axisymmetric data for Einstein equations with fixed
angular momentum. These data represent non-stationary, axially symmetric, black
holes. As a consequence, we obtain that any data in this class satisfy the
inequality $\sqrt{J} \leq m$, where $m$ and $J$ are the total mass and angular
momentum of the spacetime.
