Creatinine clearance (Ccr) is an efficient index of renal function. However, measurement of Ccr is not routinely performed for preoperative patients. To predict preoperative renal impairment, we estimated the preoperative Ccr by using serum creatinine (sCr) and other laboratory findings.702 patients served as the training samples and other 128 patients as the validation samples. (1) Values of sCr and BUN to predict lower Ccr levels were developed from the training samples. In the combination of sCr and BUN, we selected the optimal combination in receiver operating characteristics curve. This value was applied to the validation samples. (2) The predictive equation was derived from the training samples by multiple regression analysis, and it was compared with the equations previously reported.(1) The optimal sensitivity and specificity for predicting Ccr less than 30 ml x min(-1) x 1.48 m(-2) were 60% and 84%, respectively. (2) Among the equations to predict Ccr, our equation showed the highest correlation coefficient (r), and the smallest differences between predicted and measured Ccr.We determined the values of sCr and BUN to predict renal impairment and derived predictive equation. Our equation was the best to assess the preoperative renal function.