A study of placebo treatment of acute symptomatic urinary tract infection in non-pregnant women showed that about 80% obtained sterile urine spontaneously within 5 months. About one-half of these had recurrent infection within a year. Antimicrobials produced a high immediate cure rate, but only 45% maintained sterile urine for 2 years. The recurrence rate was highest during the first 2 months after treatment, and thereafter nearly constant during the subsequent 20 months. Twenty-nine percent of recurrences were recrudescences and 71% reinfections. About one-sixth of the patients had a very high recurrence rate, 2·6 infections/year, as compared with 0·32/year in the remainder. Nearly all of these patients had their first recurrence within 5 months of the initial treatment. The probability of recurrence increased with the number of previous infections. Some patients, however, after a period with many recurrences, showed a remarkable decrease in recurrence rate. If the aim of treatment is to keep periods of bacteriuria to a minimum, it is necessary to do frequent urine cultures for at least 6 months after elimination of bacteriuria.