The degree of swelling of poly(2-hydroxyethyl methacrylate) (PHEMA) is substantially increased in aqueous urea solutions relative to that in aqueous solution. In particular, the swelling of PHEMA hydrogel is markedly increased in the concentration ranges 3-4 M urea, 2-3 M methylurea, 1.5-2 M 1,3-dimethyl-urea, and 1.5-2 M tetramethylurea. Since urea could effectively expose nearly all constituents of PHEMA to the solvent, we were able to obtain a correlation between the degree of swelling and the hydrophobicity on the PHEMA hydrogel surface. The role of water molecules in transformation of the hydrogel by the addition of urea can be related to the amount of water associated with hydrophilic sites in the PHEMA hydrogel. The amount of water bound to the PHEMA hydrogel is unchanged except for a change in the concentration at which a specific transition occurs. This is due to the creation of hydrophilic sites by the extension of PHEMA residues rather than by a simple urea-PHEMA interaction. The relaxation rate and activation energy of N-14 nuclei in urea were used to evaluate the extent of the interaction between urea molecules and the PHEMA hydrogel. We found that urea does not bind to PHEMA but affects the breakdown of water structure.