Urinary tract obstruction and vesicoureteral reflux, which are often associated with urinary tract infections, may lead to progressive renal damage. Relatively little is known about the pathophysiology of this process, and a need exists for noninvasive methods of its detection in its early stages. Because urine is refluxed into the venous and lymphatic drainage of the kidney in severe vesicoureteral reflux and urinary tract obstruction, an immune response to urinary tract components might play a role in the pathophysiology of progressive renal damage and serve as a serologic marker for its presence. A solid-phase radioimmunoassay for a protein found only in the urine (Tamm-Horsfall protein [THPJ) was developed and used to measure antibody to THP in the serum of 60 subjects. Significant elevations of antibody to THP were observed in five of 15 patients with obstruction and infection of the urinary tract and in one of 10 patients with infection alone, when these patients were compared with 12 healthy control subjects. Similar elevations of antibody to THP were not seen in uninfected patients with urinary tract obstruction or in patients with low-grade vesicoureteral reflux or sepsis of nonrenal origin. These results suggest that the measurement of antibody to THP might be useful in the identification of patients with obstruction and infection of the urinary tract.