Background: The accuracy of the Cockcroft-Gault (CG) equation, the Modification of Diet in Renal Disease (MDRD) equation and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation in cancer patients was tested. An equation for estimating glomerular filtration rate (GFR) was created that was adapted to cancer patients. Methods: Data from 641 oncology patients referred for 51Cr-EDTA measurements were included. Factors that may affect GFR, i.e. gender, hypertension, diabetes, non-steroid anti-inflammatory drugs (NSAID), exposure to chemotherapy, age, weight, height, BMI, body surface area and creatinine value were correlated to 51Cr-EDTA-measured GFR. The significant factors were then included in a multiple regression analysis correlated to the measured GFR (mGFR). Results: The equation of log10 (GFR; ml/min/1.73 m2) = 2.36-0.33*log10 (age) - 0.78*log10 (creatinine) + 0.87*log10 (weight) - 0.03*NSAID*log10 (creatinine) + gender (1 = female, 0 = male)*(0.05*NSAID - 0.003*BMI) was generated. Expressed as a mean (standard deviation) of mGFR <60 ml/min/1.73 m2, this equation, the ‘New Equation', overestimated GFR by 26 (33)%, while CG, MDRD and CKD-EPI overestimated GFR by 26 (40)%, 32 (42)% and 48 (47)%, respectively. For mGFR ≥60 ml/min/1.73 m2, the New Equation underestimated GFR by only 3 (16)%, while CG, MDRD and CKD-EPI overestimated GFR by 12 (25)%, 10 (26)% and 5 (23)%, respectively. Conclusion: This study describes a novel equation with a higher accuracy than other commonly used GFR equations for estimating GFR in cancer patients.