The effect of oral oestriol (3 mg/day for 4 weeks followed by 2 mg/day for a further 6 weeks) on the vaginal bacterial flora, vaginal cytology and urogenital symptoms was assessed in a double-blind, placebo-controlled study in 35 women with symptoms of the urogenital oestrogen deficiency syndrome. No significant differences were observed with regard to the occurrence or severity of urogenital symptoms, vaginal pH, karyopyknotic index (KPI) or the baseline results of vaginal bacterial cultures in the 18 patients (mean age 71.6 +/- 1.0 years) treated with oestriol and the 17 women (mean age 72.6 +/- 1.4 years) who received placebo tablets. A decrease in both vaginal pH (P less than 0.001) and the proportion of faecal-type bacteria (P less than 0.05), and an increase in the KPI (P less than 0.01) and the proportion of lactobacilli (P less than 0.001) were recorded after 10 weeks of treatment with oral oestriol. At the end of the following 10 medication-free weeks all of these parameters except vaginal pH had returned to values that were not significantly altered from the corresponding baseline levels. In the patients treated with placebo no significant changes occurred in vaginal pH, KPI or the proportion of lactobacilli in vaginal cultures during the course of this study. Urogenital symptoms improved in both groups after medication (and even after the medication-free period) in relation to the baseline assessment, which reflects the latter's subjective nature.