The manner in which a vaginal pessary restores urinary continence is poorly understood. This report provides an explanation. Following placement of the vaginal pessary, detailed urodynamic studies in a group of 12 women with stress urinary incontinence demonstrated consistent and significant (p less than 0.005) increase in urethral functional length and urethral closure pressure under varying stressful conditions, when compared with prepessary studies. Postpessary stress testing also became normal (p less than 0.005). Prepessary and postpessary simultaneous voiding urethrocystometry and instrumented uroflowmetry demonstrated absence of obstruction to free flow of urine. Clinically, 10 of 12 patients became continent. Characteristic postpessary urodynamic alterations and Q-tip test changes provided an objective explanation that the vaginal pessary restored continence by stabilizing the urethra and urethrovesical junction to allow proper pressure transmission and by actively increasing urethral resistance to escape of urine under resting and stressful conditions.