To the Editor.
—The analysis by Dr Calvert and colleagues1 demonstrated an association between chronic occupational exposure to silica and the development of end-stage renal disease (ESRD). Based on a standardized incidence ratio (SIR) of 1.37 (95% confidence interval [CI], 0.68-2.46), the authors reported that the overall risk for ESRD was significantly elevated in gold miners who had worked underground for at least 1 year. Although this study clearly illustrates the utility of the ESRD Program Management and Medical Information System, 1 limitation of the data base is that it only identifies individuals with renal disease once they have reached end stage. As the authors point out, the exact nature of the pathogenetic link between silica exposure and renal dysfunction will only be elucidated by studies using a more moderate end point, such as the development of chronic renal insufficiency. Interestingly, Calvert et al1 showed that the risk for