The aim of this study was to characterize the differences between the prediction of GFR with Cockcroft-Gault formula (CG=(140-age)/(72 x PCr (mg/ml), for females multiplied by 0.85) and the new formula based on the multicenter trial of the Modification of Diet in Renal Diseases (MDRD=186 x P (Cr) (-1.154) x age(-0.203); 0.742 if patient is female) in elderly subjects. The study involved 100 individuals aged 65-111 years (mean age 88.3+/-14.7; 79 females and 21 males). In all subjects GFR was estimated according to both formulas mentioned above and expressed in ml/min/1.73 m2. Thereafter we calculated the difference between MDRD and CG (MDRD-CG) and analyzed its determinants in every subject. Mean GFR, obtained with MDRD was 76.0+/-24.0, whereas according to CG 67.9+/-18.6 (p < 0.0001). However, the mean MDRD-CG was up to 30.0+/-26.6 which means that MDRD results were much higher in comparison with CG. Using the multiple linear regression analysis we showed that MDRD-CG strongly depend on age (p < 0.0001), BMI (p < 0.0001) and serum creatinine concentration (p<0.0001). However, the gender has not effect on MDRD-CG value. The values of MDRD-CG strongly and positively correlated with age (r=0.7027, p < 0.0001) and negatively both with body mass index (r=-0.7171, p < 0.0001) and serum creatinine (r=-0.5590, p < 0.0001). In summary, our results show that the difference between MDRD and CG strongly depends on age, BMI and Scr. Investigators should be aware of these differences and take it into account in elderly.