The antibody-coated bacteria (ACB) test was used to localize the site of urinary tract infection in a group of pediatric and adult patients. Duration of treatment of ACB-positive (upper tract) and ACB-negative (lower tract) infections was correlated with rates of recurrence during follow-up for six or more months. Two relapses with the same organism occurred after 29 ACB-negative episodes were treated with a short course of antimicrobials (less than or equal to 14 days). Of patients with ACB-positive infections, all four given similar short-duration therapy experienced relapse with the same organism, but none of nine had relapse after long-duration therapy (greater than or equal to 28 days). High cure rates were obtained with less than or equal to 14 days therapy in ACB-negative infections and with greater than or equal to 28 days therapy in ACB-positive cases. The ACB test promises to be useful in predicting the appropriate duration of antibiotic therapy.