Open AccessObjective: The aim of this paper was to assess the relationships among chemical, phase and structural composition and etiopathogenic factors of non-infectious phosphate calculi formed in patients with low and high urinary phosphate concentrations, and to characterize the mechanism of their formation related on biochemical results. Material and Methods: Twelve samples of phosphate renal calculi were obtained, 4 from patients with low phosphaturia and 6 from patients with high urinary phosphate concentrations. Their chemical composition was determined qualitatively by energy dispersive X-ray analysis and quantitatively by spectrophotometric and thermal analysis; and their phase composition was determined by Fourier transform infrared transmission spectroscopy and X-ray diffraction. The structure of the calculi was assessed by scanning electron microscopy. Results: Non-infectious phosphate renal calculi of patients with low phosphaturia consist of poorly crystalline carbonate hydroxyapatite, whereas those of patients with high urinary phosphate concentrations consist of poorly crystalline hydroxyapatite with some amount of calcium oxalate crystals. Calculi of patients with high urinary phosphate concentrations are formed at urinary supersaturation with respect to hydroxyapatite and calcium oxalate about 4 times higher than in patients with low phosphaturia. Conclusion: In patients with low phosphaturia, the non-infectious phosphate renal calculi are formed in urine near pH 7 and contain only poorly crystalline carbonate hydroxyapatite. In patients with high urinary phosphate concentrations and hypercalciuria, the calculi are formed in urine near pH 6 and consist of both poorly crystalline hydroxyapatite and some amount of calcium oxalate crystals.