A biocompatible hydrogel comprises a specified content of polyacrylamide and pyrogen-free water. Also disclosed is a method of making the hydrogel and an injectable or implantable endoprosthesis. The hydrogel may also be used to treat a cosmetic or functional defect. Hydrogels specified according to their polyacrylamide content may be used for medical indications, such as an implantable or injectable endoprostheses for mammoplastic reconstruction, implantable or injectable endoprostheses for treating (reflux) oesophagitis, and for body contouring of various body parts.