OBJECTIVES To evaluate the relation of Tamm-Horsfall protein (THP) and citrate, both potent actors in the urinary stone forming process. METHODS Quantitative determination of THP in calcium oxalate (CaOx) stone-forming patients and healthy subjects was carried out according to the enzyme-linked immunosorbent assay method. RESULTS THP excretion in 24-hour urine samples of CaOx stone-forming patients was significantly reduced compared with healthy subjects. A significant correlation exists between the concentration of THP and citrate in the stone-forming group, as well as in the group of healthy subjects, and for the 24-hour excretion, this correlation persists in the group of CaOx stone-forming patients. CONCLUSIONS Decreased THP and citrate excretions were found in CaOx stone-forming patients. They indicate a tubular dysfunction of the distal section.