SUMMARYTo investigate the high rate of recurrence and associated bacteremia in renal transplant patients with urinary tract infection (UTI), a prospective study was undertaken in 47 consecutive renal transplant patients, using the antibody-coated bacteria (ACB) assay to define the site of infection and the type of antibiotic therapy to be prescribed. Sixteen patients (34%) developed infection in the first 3 months after transplantation, with 14 of the 16 (88%) having positive ACB tests. In contrast, 3 of 47 patients (6%) developed UTI more than 3 months post-transplantation, all with negative ACB assays.The five patients with ACB-negative infection were treated with a conventional 2-week course of antibiotics, with 100% success. The 14 patients with ACB-positive infection were treated for 6 weeks with antibiotics, with eradication of the infection in 13 of the 14 instances (94%). The one exception, a patient with chronic rejection and poor renal function, relapsed within 48 h of discontinuing therapy, with blood and urine cultures positive for the original organism. No other instance of gram-negative bacteremia occurred in these 47 transplant patients.These results suggest: (1) The majority of UTIs in the transplant begin within 3 months of transplantation, with silent or overt infection of the allograft usually present. (2) The results of the ACB assay appear to correlate well with the response to antimicrobial therapy. (3) The use of prolonged antimicrobial therapy for ACB-positive UTI appears to decrease the frequency of bacteremia and relapsing infection.