There were 5 postmenopausal women with severe recurrent cystitis who were placed on a protocol of intravaginal estrogen in an effort to control infection. Before therapy all patients had colonization of the vagina by 1 or more species of Enterobacteriaceae and an associated elevation of vaginal pH greater than 5.2 (normal range 4.0 to 4.5). After acute antibiotic treatment and continuous low dose intravaginal estrogen all 5 patients showed a return of vaginal pH to the normal range and disappearance of vaginal bacterial pathogens. Of the 5 women 4 have had no further urinary tract infection off antibiotics and on low dose intravaginal estrogen. One patient had a urinary tract infection in association with a flare in colitis, which was controlled with reinstitution of antibiotics and high dose intravaginal estrogen therapy.