More attention should be directed to upper urinary tract infections as they cause renal parenchymal damage. Recently, many reports on antibody-coated bacteria (ACB) in urological diseases have been made. We performed ACB tests in 68 cases of urological infectious diseases. The tests were made according to Thomas' method. Anti-human immunoglobulin IgG, IgA, and IgM rabbit sera were used. The results were compared using the chi-square analysis. ACB-positive were 0 approximately 15% of the cases of acute cystitis, 33 approximately 67% of the cases of chronic cystitis, 30 approximately 60% of the cases of prostatitis and urethritis, and 60 approximately 90% of the cases of pyelonephritis. The percentage was higher in the patients who had indwelling catheters than in those who did not. The ACB test was suggested to be helpful in diagnosing upper urinary tract infection in the female patients who did not have indwelling catheters. But there was the danger of making an incorrect diagnosis with this test on male patients who had indwelling catheters, and those with prostatitis or urethritis, and on female patients with indwelling catheters.