Natural history of recurrent urinary tract infections in women.

To evaluate the natural history of uncomplicated urinary tract infections in women, we observed 51 infection-prone women in a standardized fashion for a median of 9 years. During intervals when patients were not receiving antimicrobial prophylaxis, infections occurred at an average rate of 2.6 per patient-year, but the rate varied widely from patient to patient (range 0.3-7.6 episodes per year). Seventy-three percent of the observed episodes were symptomatic, with an 18:1 ratio of cystitis to pyelonephritis episodes. Infectious episodes were strikingly clustered, and rates of infection decreased in the winter months. Antimicrobial prophylaxis was highly effective in preventing acute cystitis, asymptomatic bacteriuria, and acute pyelonephritis, even when used for as long as 5 years. The proportions of infecting strains resistant in vitro to ampicillin (19%-32%) and nitrofurantoin (5%-18%) were unchanged over the 15-year observation period, while resistance to trimethoprim-sulfamethoxazole increased in the last 5 years of the study.