Elemental silicon, present as soluble silicic acid in serum and urine, has been measured by direct current plasma emission spectrometry. The method is precise and accurate, yields a standard curve that is linear up to 1000 mumol/L, and requires only a simple dilution in 10 mL/L HNO3. No spectral or background interferences have been observed from serum or urine; the absolute detection limit for silicon was 0.5 mumol/L. Silicon concentrations in serum increase by up to 20-fold in patients with chronic renal failure on hemodialysis, an increase apparently related to dietary silicon intake. No relationship with aluminum was observed in hemodialysis patients, with or without aluminum toxicity. In the undialyzed patient with chronic renal failure, the concentrations of silicon in plasma increased with decreasing glomerular filtration rate. This increase may protect renal-failure patients from possible aluminum toxicity by promoting formation of the relatively inactive aluminosilicate complex. Silicon concentrations in urine of healthy individuals exceed their serum concentrations by 20- to 100-fold. Silicon excretion in patients with renal stones was not different from that in healthy controls and showed no relationship with calcium and (or) oxalate excretion.