This prospective randomized study was undertaken to determine the efficacy of antimicrobial therapy compared with no therapy for bacteriuria in elderly ambulatory nonhospitalized women. Sixty-one women (mean age, 85.8 years) with bacteriuria were in the no therapy control group and 63 women (mean age, 85.8 years) with bacteriuria were in the therapy group; none had symptoms of urinary tract infection. One short course of antimicrobial therapy achieved a cure rate of 68.3% (43 of 63 women cured) two weeks after treatment. During the six-month follow-up period, ten (16.4%) of 61 women in the no therapy group and five (7.9%) of 63 women in the therapy group developed symptomatic urinary tract infection. At the time of six-month follow-up, 19 (34.5%) of 55 women in the no therapy group and 35 (63.6%) of 55 women in the therapy group did not have bacteriuria. We conclude that for asymptomatic bacteriuria in elderly ambulatory nonhospitalized women, short-course antimicrobial therapy is effective at two-week follow-up and that antimicrobial therapy can eliminate bacteriuria in most of these women for at least a six-month period.