In general, the invention is directed to treatment of urinary incontinence by the implantation of one or more bulking prostheses proximate to a urethral sphincter. These bulking prostheses, which may include biocompatible hydrogel, are implanted into the tissue outside the urethra, proximate to a urethral sphincter. When implanted, the bulking prostheses are in a miniature state. Upon introduction into the body, the devices enter an enlarged state. In their enlarged state, the bulking prostheses supply extra bulk to the tissues proximate to the external urethral sphincter. With the extra bulk, the patient can exercise voluntary control over the external urethral sphincter to close or maintain closure of the urethra and maintain urinary continence.