Long lasting antimicrobial activity and low toxicity are essentials for hydrogels in biomedicine. However, most reported hydrogels cannot combine these characteristics. In this work, poly (hexamethylene guanidine) hydrochloride (PHMG), a cheap cationic polymer with two terminal amino groups, was first modified with methacrylic anhydride to give PHMG dimethacrylamide (PHMGDMAAm), which was further used to prepare hydrogels with acrylamide (AAm) under ultraviolet irradiation in the presence of -ketoglutaric acid (-KGA) as photoinitiator in aqueous medium. The resultant hydrogels showed strong antibacterial activity against both Gram-negative and Gram-positive bacteria due to the PHMG segments in the hydrogel backbone. Moreover, the antimicrobial activity of the hydrogels did not decrease significantly after being soaked in water for one month and washed by water frequently for many times. Hemolysis and cytotoxicity assays demonstrated the excellent biocompatibility of the PHMG-PAAm hydrogels. This kind of low cost cationic hydrogels with long lasting antimicrobial activity and low toxicity is expected to have potential applications in biomedicine. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2027-2035