Abstract: A prosthesis is provided to replace an excised lung, comprising a hollow balloon-like structure formed generally in the shape of the lung, a filling tube, and a filling port. This prosthesis occupies the chest cavity left vacant after pneumonectomy to inhibit mediastinal shifting and overdistension and displacement of the remaining lung and other organs. This prosthesis also compliantly supports the heart and prevents the heart from contacting and adhering to other structures in the chest cavity. The volume of the prosthesis can be adjusted subsequent to implantation without subsequent surgery through a subcutaneous septal port. A combination of gases is selected to fill the prosthesis to minimize the volume change due to transfer of gas across the balloon membrane. Also provided are a method for preventing mediastinal shift and overdistension and displacement of organs following pneumonectomy using the prosthesis of this invention, and a method of making the prosthesis.