It is often difficult to distinguish acute renal failure clinically from chronic renal failure, especially in patients who do not have records of their medical history. Since fingernail creatinine (Ncr) reflects serum creatinine at the time of nail formation, it has been suggested that the level of Ncr at the distal tip of the fingernail might represent the level of the serum creatinine about four months previously. In this study, clipped fingernail specimens from 60 normal individuals, 35 patients with chronic renal failure and 33 patients with renal insufficiency with rapid onset were analyzed for creatinine by a modified alkaline picrate method. The results showed that the Ncr level of the renal insufficiency with rapid onset group (69 + or - 12.39 mu mol/100 g nail) was similar to that in the normal group (62 + or - 10 mu mol/100 g nail) and significantly lower than that in the chronic renal failure group (130 + or - 35 mu mol/100 g nail, p <0.01). The correlation between Ncr and the serum creatinine level obtained four months previously was highly significant (r = 0.895, p <0.01). This study confirms that the Ncr measurement is of clinical value in differentiating acute from chronic renal failure.