Polymeric materials based upon 2-hydroxyethyl methacrylate (HEMA) have been extensively used, especially in biomedical applications. Several reports have appeared in the literature during the past 20 years regarding the behavior of these materials upon exposure to body fluids. the results of these studies have ranged from good biocompatibility to calcification. the presence of methacrylic acid (MAA), in HEMA-based copolymers gives rise to a swelling transition at pH 7. the present study relates the swollen volume of HEMA/MAA gels to the concentrations of calcium salts at a pH where the MAA is ionized, as would occur for implanted gels. This swollen volume relationship is critical to implants which rely on dimensional integrity.