A penile prosthesis system involves the use of an inflatable penile prosthesis which is expandable under pressurization from a flaccid to an erect state. The prosthesis exhibits a low modulus of elasticity in expansion but ultimately reaches a modulus of elasticity and resists further expansion. The prosthesis may also be formed of a material which elastically expands from a flaccid to an erect state and elastically contracts thereafter. In its erect state the prosthesis is taut and the surrounding tissue may be similarly taut and pressurized because of the expansion of the prosthesis in girth. If a weakening occurs in the surrounding tissue, the tendancy to form localized penile bulges is limited because of the inability of the prosthesis to expand readily beyond its modulus transition.