A narrow therapeutic index is a characteristic feature of cytotoxic agents. Some of these agents are almost entirely eliminated renally in unchanged active form. As a consequence, assessment of the individual glomerular filtration rate (GFR) may help to predict the pharmacokinetic behaviour of cytotoxic agents in plasma more precisely. In addition, GFR-adapted individualization of cancer chemotherapy may have an enormous impact on the severity of side effects. Several methods are available to determine GFR or creatinine clearance (CrCl). GFR-measurement based on experimental methods with radiolabelled isotopes, contrast media or inulin helps to reflect the real situation very closely. In addition, 24-h urine collection is a convenient and feasible method to measure creatinine clearance. Finally, several mathematical equations exist to estimate GFR or CrCl based on serum creatinine and other parameters. Only a few of these equations have been developed in oncologic patients. However, some of these equations are routinely used in clinical practice, because they allow a rapid estimation of GFR. Based on the fact that clinically relevant differences have been assessed between calculated values and the real situation, mathematical calculation of GFR or CrCl does not seem to be appropriate to assess individual renal function precisely enough over a broad range of individual GFR or CrCl. Whether the measurement of low-molecular-weight proteins, such as cystatin C and ss-trace protein, may help to reflect the real situation more precisely is a matter of controversial debate.