A knowledge of pharmacokinetic data is essential for a rational treatment with cytotoxic drugs. The use of pharmacokinetics and insight into the metabolism make it possible to elucidate, predict or prevent toxicities after single and combined drug treatment. In addition, a careful monitoring in early clinical trials might help to find the optimal dose and schedule and to avoid less effective routes of application. This might improve the therapeutic range and efficacy of drug therapy in oncology.