We hypothesized that the increased vaginal fluid pH and altered vaginal microflora characteristic of bacterial vaginosis might predispose young women to introital colonization with Escherichia coli and to acute cystitis. To evaluate this hypothesis, we studied 291 women who presented with acute urinary symptoms for association of clinically defined bacterial vaginosis and vaginal conditions associated with this syndrome (increased vaginal fluid pH, absence of lactobacilli, and abnormal vaginal fluid gas-liquid chromatographic patterns) with E coli introital colonization and urinary tract infection. Escherichia coli introital colonization and urinary tract infection were both significantly more frequent among women with a high vaginal fluid pH, an absence of vaginal lactobacilli, or an abnormal vaginal fluid gas-liquid chromatographic pattern characteristic of bacterial vaginosis. Escherichia coli introital colonization was also more frequent in women with bacterial vaginosis. These associations and an association of bacterial vaginosis and E coli urinary tract infection were strong only among the 144 women who were diaphragm users. We conclude that bacterial vaginosis, or an altered vaginal microflora as reflected by an abnormal gas-liquid chromatographic pattern characteristic of bacterial vaginosis, is associated with E coli introital colonization and acute symptomatic urinary tract infection in women who use diaphragms.