We present results from extensive molecular dynamics simulations of collapse transitions of hydrophobic polymers in explicit water focused on understanding effects of lengthscale of the hydrophobic surface and of attractive interactions on folding. Hydrophobic polymers display parabolic, protein-like, temperature-dependent free energy of unfolding. Folded states of small attractive polymers are marginally stable at 300 K and can be unfolded by heating or cooling. Increasing the lengthscale or decreasing the polymerwater attractions stabilizes folded states significantly, the former dominated by the hydration contribution. That hydration contribution can be described by the surface tension model, ⌬G ؍ ␥(T)⌬A, where the surface tension, ␥, is lengthscale-dependent and decreases monotonically with temperature. The resulting variation of the hydration entropy with polymer lengthscale is consistent with theoretical predictions of Huang and Chandler [Huang DM, Chandler D (2000) Proc Natl Acad Sci USA 97:8324 -8327] that explain the blurring of entropy convergence observed in protein folding thermodynamics. Analysis of water structure shows that the polymer-water hydrophobic interface is soft and weakly dewetted, and is characterized by enhanced interfacial density fluctuations. Formation of this interface, which induces polymer folding, is strongly opposed by enthalpy and favored by entropy, similar to the vapor-liquid interface.dewetting ͉ folding ͉ hydration entropy ͉ hydrophobic hydration ͉ hydrophobic interaction H ydrophobic interactions are one of the major contributors to biological self-assembly in solution, including protein folding and aggregation, micelle and membrane formation, and biomolecular recognition (1-5). Recent work in this area has focused on the lengthscale dependencies of hydrophobic hydration and interactions (4,(6)(7)(8)(9). In particular, a recent theory by Lum, Chandler, and Weeks (6) highlighted the different physical mechanisms of solvation of small and large hydrophobic solutes in water. Small solutes are accommodated in water through molecular-scale density fluctuations (10, 11), whereas solvation of larger solutes requires formation of an interface similar to that between a liquid and a vapor (4,6,12). This change in physics is also reflected in thermodynamic (entropy vs. enthalpy dominated hydration) (9) and structural (wetting vs. dewetting of the solute surface) (4, 12, 13) aspects of hydration. Similarly, interactions between larger hydrophobic solutes in water (14-18) are characteristically distinct from those between their molecular counterparts (19,20).The differences between the hydration and interactions of small and large solutes characterize many-body effects in hydrophobic phenomena. Effects of similar origin are also at work in association of small hydrophobic solutes into a larger aggregate (21,22) and are quantified by the n-particle potential of mean force (PMF) (23-26). For n Ͼ 3, however, the dimensionality of the system makes calculations of n-particle PMFs computa...