Forty-one adult women with acute lower urinary tract infections (UTI) were randomly treated for three days with norfloxacin or trimethoprim/sulfamethoxazole (TMP/SMX). Infection was eradicated in 100% of norfloxacin-treated patients and in 95% of TMP/SMX-treated patients. UTI recurred in 29% of patients treated with norfloxacin and in 41% of those treated with TMP/SMX. Post-therapy vaginal administration of lactobacillus suppositories resulted in a recurrence rate of UTI of only 21%, while in patients given sterilized skim-milk suppositories the recurrence rate was 47%. This study indicates that lactobacillus vaginal suppositories are safe and may be effective in reducing the recurrence of UTI following antimicrobial therapy.