INTRODUCTION In lung cancer patients treated with chemotherapy, renal function is an important parameter to be monitored. Since measurement of renal function with either isotope or creatinine clearance is time consuming and expensive, we evaluated which of the following equations: Cockcroft-Gault (CG), Wright, modification of diet in renal disease equation (MDRD), MDRD adjusted for body surface area (BSA) and chronic kidney disease epidemiology collaboration (CKD-EPI) best resembles endogenous creatinine clearance (ECC) and could therefore replace its measurement in clinical practice. METHODS 218 lung cancer patients, who had their 24-h creatinine secretion in urine measured prior to the start of any chemotherapy, were included. Estimation of renal function was calculated and compared to ECC. RESULTS There were no major differences in the performance of the tested equations. Mean percentage error of more than 20% and general underestimation was common to all equations. Wright equation performed best although it describes only 43% of ECC variability. Mean measured ECC was 94 mL/min (95% confidence interval [CI]: 90-98 mL/min) and 90 mL/min for Wright equation (95% CI: 87-93 mL/min) (Supp. Fig. 3). MDRD and CKD-EPI equation performed poorest since they do not include any body size descriptor. Large deviations of differences were observed, with a median standard deviation of more than 20% and deviations from ECC exceeding 100%. Wright equation performed best, whereas, despite their leading role in the detection of renal diseases, the MDRD and CKD-EPI equation performed poorest since they do not include any body size descriptor. In the range of ECC<50 mL/(min×1.73 m(2)), the CG equation most often detected a contraindication for cisplatin use. Differences between ECC and calculated values correlated with patients' weight, BSA and body mass index when these were not included in the equation itself. CONCLUSIONS In evaluating the renal function of lung cancer patients, equations adjusted for body size descriptors should be preferred. Estimated renal function should be interpreted against the characteristics of patient's body size and special attention is needed when these are reaching the extremes.