Hydrogels that contain phosphate groups in side chains were studied for their usefulness in drug-delivery soft contact lenses (SCLs). Naphazoline, a model drug having a cationic group, was incorporated into an SCL because of its phosphate groups and was released over a period of about 14 h. For the SCL, the naphazoline content was equivalent to the phosphate group content. It is suggested that drug-delivery SCLs can be designed to contain the needed amount of a drug through the choice of the ionic group for the ligand. Furthermore, the SCL having amide groups and phosphate groups had high transparency and an unchanged shape. It it; suggested that amide groups and phosphate groups must be introduced into the polymer in equimolar amounts to give the necessary polymer-drug interaction. Therefore, hydrogels having a drug-delivery system were synthesized by the inclusion of a phosphate group and an amide group. These hydrogels are also applicable to SCL materials. (c) 2005 Wiley Periodicals, Inc.