We report the recent development of a theoretical method to calculate the protein configurational entropy in explicit solvent from statistical properties of the solvent-averaged protein potential energy surface. This method can be implemented by combining molecular simulation and integral-equation theory of liquids. Our method does not assume Gaussian distribution of protein configurations, and can be applied to unfolded or misfolded states of protein in which an average protein structure is not well defined. An illustrative application is made to misfolded state of 42-residue amyloid beta protein in water. (C) 2011 Elsevier B.V. All rights reserved.