OBJECTIVES: Utility index are a basic tool to calculate the “Quality Adjusted Life Years-QALYs” and the derived value of cost per QALY. At present there are some models used to calculate these measures from preference studies. Some of them, as it is the EQ-5D, include a dummy variable which could enlarge the utility index value when score in all its dimensions are the best possible. So, validity of those tools may be affected. The aim of present study is to analyze the validity of Spanish version of the EQ-5D as an assessment instrument of preferences. METHODS: Various simulations were carried out with different samples which had the same mean value in the EQ-5D tariff, but different percentage of subjects who achieved the best score (1 point) after an intervention. It was understood that the intervention produced an improvement of 1 point in one of the EQ-5D dimensions, and that benefit was present in all subjects. RESULTS: Different samples obtained different values in the EQ-5D after intervention depending on the percentage of subjects who achieved the best value in all the EQ-5D dimensions. Maximum differences were of 0.15 points, which is a value higher than that considered clinically relevant for the EQ-5D. CONCLUSION: An improvement in an utility index associated to an intervention could depend on the characteristics of the studied sample and this do not seem adequate or desirable. Although an intervention is equally effective in various samples, the results are not the same and this could be a serious handicap for the utility index. Other methods to calculate utility index values must be developed in order to better evaluate the changes associated to an intervention.