To the Editor In their Research Letter, Thomas et al1 report an increased risk of first-episode renal stone in Swedish men who took ascorbic acid supplements (but no other supplements). This risk is almost certainly overestimated. Cases were identified according to code N20 of the International Statistical Classification of Diseases: calculus of kidney or ureter in the absence of hydroureter (www.cdc.gov/nchs/data/dvs/2006-Vol-I.pdf). Contrary to what the authors claim, the symptoms of renal colic are often nonspecific, especially in the absence of hydroureter2,3, and for this reason, nonspecific abdominal or flank pain frequently prompts an investigation for renal stone.2,3 As many as 40% of renal stones identified by means of imaging studies are asymptomatic4—not surprisingly, since the prevalence of renal stone in asymptomatic men is 10%.5 The fact that physicians and the general public are repeatedly warned that ascorbic acid supplements cause renal stones (even though the evidence is inconclusive) predictably lowers the threshold for testing for them. Thus, all other things being equal, takers of high-dose ascorbic acid supplements are much more likely than nontakers to be investigated for renal stone, artificially increasing the number of code N20 cases linked to vitamin C. These factors, combined with the authors’ sole reliance on registry entries for case identification, strongly expose their study to the important epidemiological error of “confounding by indication.”