A calculation of the long-range interaction energies between two undulating neutral lipid bilayers was performed. At bilayer spacings above several nanometers, the bilayer interaction can be considered to be composed solely of an attractive van der Waals component plus a repulsive undulation component arising from the entropic confinement of thermally excited out-of-plane motion of the bilayers by neighboring bilayers. The objective of this work was to determine if the long-range swelling behavior that has been observed in neutral multilamellar lecithin systems (e.g., bilayer spacings as large as 150 Angstrom), can in fact be explained by the undulation repulsion, which is the currently accepted explanation. It was found that the relative magnitudes of the long-range van der Waals and undulation energies ale essentially determined by the magnitude of the dimensionless ratio A(H)K(b)/(kT)(2), where A(H) is the Hamaker constant, K-b is the elastic bending modulus of the bilayer, and kT is the thermal energy. When this value is much greater than 1, the van der Waals attraction dominates at large bilayer spacings, while the opposite is true when this ratio is much smaller than 1. Using the measured physical parameters for lecithin systems, the value of the dimensionless ratio is over 50 and the long-range retarded van der Waals interaction remains well over order of magnitude greater than the undulation repulsion. This difference is far too large to be explained by uncertainties in the values of the key system parameters and suggests that another mechanism is responsible for the swelling of the lecithin bilayers.