It has been shown in a recent experimental study for an alanine-based peptide that the transition from the coil state to the helix state occurs at high pressures. Here we show that this result can be elucidated by our method wherein the solvent entropy, which is calculated using an elaborate statistical-mechanical theory, is treated as an essential factor. The structural stability is described by the competition between the solvent-entropy gain and the conformational-entropy loss of the polypeptide upon the transition. Though the former is smaller at low pressures, it prevails over the latter at high pressures. (C) 2009 Elsevier B.V. All rights reserved.