There are doubts about whether the values obtained from the Cockroft-Gault (Cl(CG)) and Modification of Diet in Renal Disease (GFR(MDRD)) formulas are comparable to the more traditional formula used to obtain the creatinine clearance from a 24-hour urine collection (ClCr(m)), particularly in patients with only one kidney. The present study aimed to compare these formulas in individuals with one remaining kidney after previous nephrectomy (Nx) and to verify which estimated formula correlates more closely with ClCr(m). Thirty-six patients who had undergone Nx had their renal filtration analyzed with Cl(CG), GFR(MDRD) and by ClCr(m). The average time after Nx was 11.6 ± 9.0 years, and the average age at the time of the study was 50.7 ± 10.6 years old (X ± SD). The results of three clearances were 81.1 ± 35.6 mL·min·m(2) for ClCr(m), 70.4 ± 24.0 mL·min·m(2) for ClCr(CG), and 71.2 ± 19.2 mL·min·m(2) for GFR(MDRD) (with ClCr(m) > ClCr(CG) and GFR(MDRD); P < .001). No difference was found between the ClCr(CG) and GFR(MDRD) values (P = .72). The data demonstrated that both estimate formulas were strongly correlated with ClCr(m), although ClCr(CG) was more closely associated with ClCr(m) than GFR(MDRD) (ClCr(CG) with r(2) : 0.64 and GFR(MDRD) with r(2) : 0.34; P < .001). In conclusion, for people with only one kidney remaining after NX, our data showed that glomerular filtration rate estimation by ClCr(CG) is more related to the values obtained with the traditional clearance measurement based on a 24-hour urine collection test.