A test for antidoby-coated bacteria in the urine for the diagnosis of the anatomical level of urinary tract infection was evaluated in an unselected series of patients with urinary tract infection as a complication of other diseases of the uro-genital tract. Of 19 patients with cancer of the bladder, prostata and the kidney who had clinical signs of lower tract infection, 14 had antibody-coated bacteria in the urine. Among adult patients without cancer of the urinary tract but a clearcut history of upper tract infection, 17 had antibody-coated bacteria, while 8 had not. Six patients also without history of cancer, with clearcut lower tract infection, had antibody-coated bacteria; 38 had not. The results indicate that cancer of the urinary tract seems to influence the correct establishing of level of infection by this method. In patients with diseases of the urinary tract in additional to infection, antibody coating by bacteria in the urine must be interpreted with caution.