In this work we describe Monte Carlo simulations of a fluid confined between planar walls. We derive a free energy difference method (FEDM) in the isotension ensemble. We illustrate the method by varying the separation between the walls at constant chemical potential. It is shown that the FEDM is a viable alternative to grand canonical ensemble (GCE) simulations. Our method appears to be less susceptible to the difficulties that confront the simulation of confined fluids at high density in the GCE.