Autoantibodies to the Tamm-Horsfall (TH) protein were analyzed in sera from 116 patients with pyelonephritis. The increases in antibody levels were limited in 23 patients with radiological detected renal damage during 31 attacks of acute pyelonephritis. 8 children with abnormally low total and/or unilateral 51Cr EDTA clearance had significantly lower IgG antibody levels to TH protein, than 14 children with normal clearance rate. All 61 children with renal damage had significantly low IgG, IgA and IgM antibody levels to TH protein 6 months after last infection as compared to the reference group. For IgG antibodies, the mean was well below -2 S.D. 12 children with increased serum creatinine had significantly lower IgG antibody levels than those with