Studies of patients with hyperuricosuria have provided four kinds of evidence linking their hyperuricosuria to calcium stone formation and identifying them as a distinct entity apart from other calcium stone formers. The evidence includes a more virulent natural history of stone disease, in vitro evidence for heterogeneous nucleation of calcium oxalate crystallization by seed crystals for uric acid or sodium hydrogen urate, and a reduction by uric acid or urate of the inhibitory activity which urine normally has upon calcium oxalate crystal growth. The fourth bit of evidence, the dramatic reduction in stone formation rates with allopurinol treatment, will be discussed in detail in a following paper.