Hypercalciuria is common in patients who form calcium oxalate urinary stones and is considered by many to be the cause ofthe disorder. This review shows that there is little relationship between either the rate of stone-formation or calcium oxalate crystalluria and the urinary excretion of calcium. There is, however, a strong relationship between these parameters and the urinary excretion of oxalate which is slightly, but significantly, elevated in stone-formers compared with normals. It is concluded that this mild degree of hyperoxaluria may be much more important than hypercalciuria in the genesis of calcium oxalate stones.