Combination chemotherapy plays an important role in anticancer chemotherapy. Many new anticancer drugs, including target-based drugs, have been introduced clinically. The opportunity to evaluate combination chemotherapy including new drugs has increased recently. Combination phase I/II studies must be designed scientifically and ethically in order to evaluate the efficacy and safety of combination chemotherapy including new drugs. It is important to choose appropriate drugs and to determine an effective method of administration, because drug-drug interaction can be a problem in combination chemotherapy. It is important to clarify the causes of drug-drug interactions through pharmacokinetics analysis because these interactions influence toxicity and efficacy. To conduct high quality clinical trials, we need to improve the infrastructure of such trials.