I had this argument with my primary physician, who's a nephrologist, recently. I went to PubMed and searched for papers on renal calculi and ascorbate. I did not find a single paper with data supporting causation. There is only a very small number--two or three--of case studies but the patients are in ill health already and it is difficult to make a causal link between their quite modest ascorbate levels and stones. I did find published arguments providing a plausible causal link ascorbate->oxalate->renal cacluli, but then the data showing strongly elevated oxalate are missing. The only data are showing modest rises in oxalate at modest doses of ascorbate. Orthomolecular doses have not been well studied but it appears that the large doses of ascorbate only cause modest elevations in oxalate as well. My conclusion is that it is very unlikely that consumption of large quantities of ascorbate will lead to kidney stones. You can't say never, but the risk appears to be very small unless one is a stone-former already. But then a stone-former already has to avoid healthy foods, such as spinach, that contain higher levels of oxalate, too.