  The housekeeping heat $Q\hk$ is the dissipated heat necessary to maintain the
violation of detailed balance in nonequilibrium steady states. By analyzing the
evolution of its probability distribution, we prove an integral fluctuation
theorem $\mean{\exp[-\beta Q\hk]}=1$ valid for arbitrary driven transitions
between steady states. We discuss Gaussian limiting cases and the difference
between the new theorem and both the Hatano-Sasa and the Jarzynski relation.
