A crosslinked poly(allylamine) was prepared by copolymerization of allylammonium dihydrogen phosphate with N,N'diallyl-1,3-diaminopropane bis(dihydrogen phosphate). This polymer has both a high phosphate adsorption capacity and a low degree of swelling. Thus, the polymer should be useful as a therapeutic agent for hyperphosphatemia, and compared with sevelamer hydrochloride, should have fewer side effects such as constipation.