Accurate differentiation of upper from lower urinary tract infection (UTI) has been reported based on the presence or absence of antibody-coated bacteria (ACB) in the urinary sediment. The specificity of ACB localization in adults has been reported to be high based on a strong correlation with localization results obtained with the bladder washout technique. The present study examines the efficacy of ACB testing with respect to accurate anatomic localization of UTI in 20 studies in 14 patients exhibiting bacteriuria as a result of chronic urethral catheterization. 17 studies (85%) demonstrated significant ACB. Localization of UTI was assessed in each study by the bladder washout technique. Following instillation of a washout solution, a normal decrease in colony counts was observed in only 3 studies (15%), a frequency significantly lower than the value of 90% previously reported in noncatheterized bacteriuric patients (p < 0.001). Each of these 3 patients was ACB-positive, yet anatomic localization indicated lower tract UTI. The observed characteristics of the 14 patients included the absence of upper tract signs and symptoms, and spontaneous clearing of ACB and bacteriuria following catheter removal. These findings suggest that in chronically catheterized patients, both (1) the observed abnormal resistance of bacteria to bladder washout and (2) the high frequency of positive ACB tests are indices of severe chronic tissue invasion and intense antibody production in the lower urinary tract in this condition and thus not of value in localizing infection to the upper tract. Neither ACB nor washout testing with current techniques can reliably localize UTI in chronically catheterized patients.