The paper proposes a way to analyze the entropic contribution to the balance of intermembrane forces. The relationship between the amplitude of out-of-plane fluctuations of lipid molecules U-T at different intermembrane distances and the decay length lambda of entropic forces has been derived. It is shown that the value of these fluctuations U-T, calculated in accordance with the theory of J. Israelachvili and H. Wennerstrom (Langmuir, 1990, 6, 1873-876), only depends on the intermembrane distance D and the decay length lambda, the parameters determined in the experiment. Moreover, with the increase of the distance D the value of these fluctuations is rapidly approaching the value of the decay length lambda (for instance, at D = 8 Angstrom and lambda = 1 Angstrom, U-T = 0.97 Angstrom). The paper is offering an analysis of the determination of the amplitude of lipid molecules U-T from neutron and X-ray diffraction data. If entropic forces are dominating, the values of U-T, determined from neutron and X-ray measurements and those predicted by the theory must be the same. The known neutron and X-ray diffraction data (J. Mol. Biol. 1979, 134, 673-691. Biophys. J. 1992, 61, 434-447) are being discussed. It is shown that they confirm the existence of out-of-plane thermal fluctuations of lipid molecules, which are, at least in the liquid phase, sufficient to contribute significantly to intermembrane short-range repulsion.