Recovery of renal function after acute renal injury is an important clinical determinant of patient morbidity and mortality. However, studies covering this field are scarce and nonhomogeneous. The "gold standard" for monitoring kidney function is to measure "true" glomerular filtration rate (GFR) by 24 h urine isotope collections. Serum creatinine is a practical surrogate for GFR in the daily caring of the patients. Unfortunately, use of serum creatinine is limited by several patient dependent and independent factors. Also, serum creatinine concentration may fall to one-third of normal in advanced CKD secondary to its extrarenal clearance. The accuracy of serum creatinine in determining kidney function can be improved by serial measurements of serum creatinine, assessment of the reciprocal slope of the serum creatinine, or serial measurements of 24 h creatinine clearances. However, it is difficult to decide residual renal function in AKI patients requiring renal replacement therapy. Furthermore, even when the changes of kidney function is linear, there is a poor relationship between [Cr]-1 and the rate of rate function recovery, as demonstrated by direct comparison with urine isotope clearances. Moreover, even 24 h creatinine clearances yield poor estimates of renal function recovery.