Urinary incontinence (UI) is the involuntary leakage of urine. The prevalence of UI varies by age and definition and has been estimated as impacting approximately 25% of young women, 13% to 47% of middle-aged and postmenopausal women, and 20% to 45% of older women. UI is also a large component of health care expenditure, with the direct cost of care for UI in the United States (US) estimated annually at $12.4 billion (2001 US dollars) for women alone, and $19.5 billion (2000 US dollars) for both men and women.