The juxtaposition of economic and clinical evaluation raises new issues in the design of clinical trials. Recent pharmacoeconomic guidelines provide some direction, but do not deal with the appropriate timing of economic evaluations in the drug developmental process. Ideally, pharmacoeconomic data should be available at the time of the regulatory and formulary decision making. Current pivotal phase III trials do not provide these data; they are designed to test safety and efficacy (does the drug work under optimal circumstances?) and not to answer questions about the effectiveness of a drug, the more relevant question for economic analysis (does the drug work in usual care?). The use of more "naturalistic" designs for some phase III randomised trials has been suggested. These so called "effectiveness trials" more closely reflect routine clinical practice. They use a more flexible dosage regimen, and a "usual care" instead of a placebo comparator. Patients randomised are more representative of actual practice and outcomes include quality of life and utility measures. They are more suited to provide the data needed to estimate the real benefit of the treatment in actual care. When costs are applied and compared with these benefits, you can estimate the efficiency of allocating resources to this new drug. Increasing the use of effectiveness trials in phase III would decrease the need for economic modelling.