The solubility parameters of pure poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(2-hydroxyethyl methacrylate/itaconic acid) [P(HEMA/LA)] hydrogels were determined by 20 solvents with various solubility parameters in swelling experiments. The solubility parameter of pure PHEMA was 26.93+/-0.46 (MPa)(1/2). The effect of mole percentages of itaconic acid (IA) in P(HEMA/IA) hydrogels on the solubility parameter was investigated. The measured values were compared to literature and solubility values theoretically determined by group contribution values of van Krevelen and Hoy. The incorporation of IA into the hydrogel system slightly increased the solubility parameter.