The effect of the membrane‐membrane interactions in multilayer membrane systems on the free energy is investigated. An approximation by which the free energy increment &Dgr;finduced by the interactions is expressed in terms of the membrane‐membrane distance distribution function is introduced.The analytic expression of the distance distribution function is derived on the basis of the Monte Carlo calculation. It is found that &Dgr;fbehaves as &Dgr;f∼ &rgr;4for the small membrane density &rgr;. © 2004 American Institute of Physics