Objective:To investigate the characteristics of renal papillary calcareous deposits and their role in the formation of calcium oxalate kidney stone,and to explore the formation mechanism of renal calcareous deposits.Methods: A total of 21 patients with calcium oxalate kidney stones were included in the present study.The components of the calculi were detected by Fourier-transform infrared spectroscopy.The calcific plaques were observed and the renal papilla biopsy specimens were obtained during PCNL.The specimens were then subjected to alizarin bordeaux staining and light microscopic examination.The expression of osteopontin,BMP-2, and type Ⅱcollagen in the kidneys was examined by immunohistochemistry.Seven resected renal specimens from patients with non-urolithiasis served as control.Results: Renal papillary calcific plaques were found in all the 21 patients with renal calculi.Local calcareous deposits were found in the renal interstitium around tubular basement membrane and extended toward the mucous membrane in the renal papillas.Microscopically,once the calcareous deposits pierced into the collection system,tiny stones could be seen growing on the deposits.Immunohistochemistristry showed the renal tissues of renal stone patients and normal renal tissues both expressed osteopontin,but not BMP-2 or typeⅡ collagen.Conclusion: Renal papillary calcareous deposits may be one of the initiating nidi for kidney stone formation and they may not be an osteoblastic reaction resembling arteriosteogenesis.