Clinical use of fingernail creatinine estimation to predict duration of azotemia is yet to be validated. We studied the fingernail creatinine concentrations in 48 subjects: seven controls, nine with acute renal failure, five with rapidly progressive glomerulonephritis, 12 with chronic renal failure and 15 with end-stage renal failure on maintenance hemodialysis. The creatinine concentration in aqueous eluates of powdered nail clippings was determined by the alkaline picrate reaction. The mean fingernail creatinine concentration was significantly higher in patients with chronic renal failure (93.7 +/- 83.7 micrograms/g) and end-stage renal disease on maintenance hemodialysis (118.4 +/- 46.8 micrograms/g) as compared to those with acute renal failure (36.6 +/- 23.7 micrograms/g) and rapidly progressive glomerulonephritis (35.8 +/- 20.6 micrograms/g). The creatinine concentrations did not differ significantly between normal subjects (27.2 +/- 28.7 micrograms/g) and those with acute renal failure and rapidly progressive glomerulonephritis. However because of large variability in the values of fingernail creatinine concentrations within each group, the test lacked specificity. Therefore, this investigation is an unreliable indicator of duration of azotemia in individual patients and is not likely to be of much clinical use.