Background: Non-surgical treatment for stress urinary incontinence (SUI) is recommended as first-line therapy, yet few prospective studies and no randomized trials compare the most common non-surgical treatments for SUI. Purpose: To present the design and methodology of the ambulatory treatments for leakage associated with stress (ATLAS) trial, a randomized clinical trial comparing three interventions for predominant SUI in women: intravaginal continence pessary; behavioral therapy (including pelvic floor muscle training and exercise and bladder control strategies); and a combination of the two treatments. Methods: Treatment outcome measures, collected at 12 weeks and six and 12 months post randomization, include the Patient Global Impression of Improvement (PGI-I), the Stress Incontinence Scale of the Pelvic Floor Distress Inventory (PFDI), seven-day bladder diaries, Pelvic Floor Impact Questionnaire (PFIQ), Pelvic Organ Prolapse-Urinary Incontinence Sexual Function Questionnaire (PISQ-12), Patient Satisfaction Questionnaire (PSQ) and the Medical Outcomes Study Short Form Health Survey (SF-36). Limitations: The study design reduces most common biases, but some degree of selection bias may remain. Conclusion: This trial will provide useful information to help counsel women with stress and mixed incontinence about the relative efficacy and satisfaction with pessary, behavioral therapy and both treatments combined. © Society for Clinical Trials 2007.