Stones from the upper urinary tract of 172 male and 73 female patients have been analyzed by X-ray diffraction. Pure calcium oxalate stones contribute 39.5% of all stones from men, but only 6.9% of stones from women. This difference is consistent both above and below 50 years of age and largely independent of whether patients with known causes of stone formation are excluded. Other sex differences, such as a greater tendency to pure calcium phosphate and triple phosphate stones in female patients, are to a large extent dependent on demonstrable causes. The possibility of analysis error is discussed, considering the possibility of minor amounts of calcium phosphate being overlooked in “pure” calcium oxalate stones. Although this possibility cannot be rejected, there will still be a definite male preponderance of calcium oxalate stones with no or minimal admixture of calcium phosphate.