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OBJECTIVE: To discuss the calculation and application of confidence intervals in pharmacoeconomic studies. DATA SYNTHESIS: The increasing frequency with which pharmacoeconomic evaluations are made within clinical trials makes it possible to obtain information on the outputs and costs of an intervention in each patient of a sample under study. This allows the same statistical principles commonly used in clinical trials to be applied to cost or cost-effectiveness data. The methodology described in this article would allow expression of cost-effectiveness ratios in the form of confidence intervals. The calculation of the cost-effectiveness ratio by means of a confidence interval may have important practical consequences, both in decision-making on the choice of 1 intervention versus another and in calculating the size of the sample necessary to identify statistically significant differences, from both clinical and economic points of view. CONCLUSIONS: The complementary use of confidence intervals and sensitivity analysis makes it possible to measure uncertainty related and unrelated to variability in sample data, allowing the decision to adopt 1 technology or another to be based on the most objective information available. Although several ethical and methodologic concerns remain to be addressed, this methodology may contribute to improving the more rational and efficient use of drugs.