Urine culture performed during and after antimicrobial therapy will differentiate unresolved urinary tract infections from recurrent infections. Recurrent infections with the same organism and at close intervals are frequently caused by a focus of bacterial persistence within the urinary tract, and infections with different organisms and/or at longer intervals are characteristic of reinfections with bacteria from outside the urinary tract. Unresolved infections are usually due to resistant bacteria and are treated by modification of therapy based on antimicrobial sensitivity testing. When unresolved bacteriuria is caused by organisms sensitive to the initial antimicrobial therapy, azotemia or a large bacterial mass density should be suspected. Recurrent infections at close intervals and/or with the same organism are usually caused by a bacterial focus in an acquired or congenital abnormality of the urinary tract such as infection stones, which must be removed to cure the recurrent infections. If the bacterial focus within the urinary tract cannot be removed, long-term low-dose antimicrobial prophylaxis can prevent the morbidity of recurrent infections. Reinfection requires careful bacteriologic monitoring and low-dose prophylactic, intermittent, or post-intercourse antimicrobial therapy.