The mechanism by which estrogen supplementation improves the symptom of stress incontinence in postmenopausal women is unclear. Six women with proved premature ovarian failure were studied urodynamically before and after administration of oral and vaginal estrogen to study estradiol's effects on lower urinary tract function. Regardless of the mode of administration, estrogen supplementation did not produce any significant change in urethral pressure, functional length, or cystometric parameters. However, a significant increase in pressure transmission ratio (P less than .05) to the proximal and mid-urethra was noted after the administration of vaginal estrogen cream. We conclude that estrogen alone, in the absence of aging and other known precipitating factors for stress incontinence, is of minimal significance in maintaining normal urinary tract physiology.