Objective: Mid-urethral slings (MUS) are increasingly common surgical procedures for the treatment of stress urinary incontinence (SUI) in women. There are currently no adequately powered trials with sufficient length of follow-up comparing the efficacy or safety of the transobturator and retropubic MUS. As a result, no selection criteria are available to guide surgeons or patients. This article describes the methodology and rationale for the Trial Of Mid-Urethral Slings (TOMUS). Patients and Methods: The primary aims of this randomized controlled trial are to compare subjective and objective success rates for urinary incontinence (UI) at 12 and 24 months following retropubic and transobturator MUS procedures. Secondary aims are to compare the resolution of overall and stress-specific UI, morbidity, the time to adequate voiding, satisfaction, and quality of life in the 2 groups. TOMUS will also assess the clinical utility of pre-operative urodynamics in women undergoing MUS procedures. The primary outcome will be obtained at 12 months and 24 months. The definition of treatment suc cess is 2-fold. Objective treatment success is defined by a negative stress test, a negative 24-hour pad test, and no retreatment for SUI. Subjective treatment success is defined by no self-reported leakage in a 3-day diary, no self-reported SUI symptoms, and no retreatment for SUI. Enrollment began April 2006 and is expected to be complete in 2 years. Conclusions: The TOMUS trial is designed to provide outcome and safety information to pelvic surgeons and their patients on the 2 most commonly performed MUS techniques.