SummaryDuring a urinary infection, antibodies are produced against a variety of antigens both host and bacterial. Some of the antibodies are protective in animal-models. This is the case with the anti-pili, anti-K and anti-O antibodies. Others, like the anti Tamm-Horsfall protein auto-antibodies, could be responsible for renal scarring. Antibodies production is generally more marked in pyelonephritis than in cystitis. In the serum, the antibodies are of the IgG and IgM types and in the urine of the IgA and IgG types. This qualitative difference is the basis of diagnostic tests for localizing the level of urinary tract infection. The antibody-coated-bacteria test seems to be the easiest routine test. The importance of this test for the diagnosis of urinary infection in the children as well as in adults (especially in pregnant women) is discussed. Furthermore, a negative test is an accurate predictor of therapeutic success using a single dose antibiotic regime. The measurement of antibody titres against the Tammhorsfall protein is potentially of value in the long term follow-up of patients presenting with acute pyelonephritis.