Monte Carlo simulations of hydrated alpha-cyclodextrin are made to analyze the properties of the water molecules inside the hydrophobic cavity. An average of five water molecules is found in the cavity, where only 2.4 are hydrogen bonded to the alpha-CD. The hydrogen-bond interactions between the alpha-CD and the inner water molecules are reduced when compared with those outside the cavity. The free energy of binding one water molecule inside the hydrated alpha-CD is obtained using the thermodynamic perturbation theory. From these the entropy contribution is obtained to ascertain the hydrophobic strength of the cavity. (c) 2005 Elsevier B.V. All rights reserved.