The authors report a case of pure ammonium urate stone. It was a very large lightly radiopaque pelvic stone, which was extracted by pyelotomy and analysed by infra-red spectrography. The clinical history did not reveal a long history of chronic infections. The stone was diagnosed by urography performed after an acute urinary trad infection. Proteus mirabilis was found at one time, but only after the surgical operation. The blood and urinary laboratory check-up was normal. No case of such a large pure ammonium urate stone especially in adults, can be found in the literature. The pathogenesis is not obvious, and could not be explained without an abnormal urinary ammonia production during the development of the stone, but the lack of associated phosphate precipitation cannot be explained.