Objectives:To investigate the presence of calcium urate, sodium, potassium and calcium in 'pure' uric acid calculi. Methods: Ten spontaneously passed uric acid urinary calculi have been examined by stereoscopic microscopy, infrared spectroscopy, scanning electron microscopy and energy dispersive X-Ray analysis. Results: The analysis of selected uric acid calculi revealed the presence of a near-pure calcium urate in two cases and of calcium enriched urate zones in all of the samples. Furthermore, in some cases complex urates containing sodium, potassium and calcium in different proportions appeared on the surface of the uric acid calculi studied , potassium being generally predominant. Ammonium urate was not detected. Conclusions: Most urinary uric acid calculi are not pure in composition. 'Complex urates', sodium, potassium and calcium were found together in various proportions in many areas of uric acid stones.