OBJECTIVES: Some European health authorities such as the National Institute of Clinical Excellence in the UK have published Health Technology Assessment (HTA) guidelines, which recommend the use of the Quality Adjusted Life Years (QALYs) outcome measure as the reference case. The ECHOUTCOME project is an interdisciplinary European research platform funded by the seventh Framework Program of the European Commission which objectives are to assess the validity of the QALYs for its potential use in cost effectiveness analysis (CEA) in European countries and proposing new European guidelines for conducting CEA studies. METHODS: Over a period of 3 months, a total of 1,200 students from Belgium, France, Italy and the UK answered hypothetical health states in which the health states and time in a given health states were varied in order to test for: von Neumaniann-Morgesten assumptions, mutual independence in utility, and the relevance of the multi-linear multi-attribute utility theory, which are the basis for the QALY calculation, as currently performed in the HTA literature. RESULTS: The preliminary findings of the experiment provided strong evidence that utilities obtained by varying the health states and the duration of a given health state fail to comply with the theoretical basis of the QALY. CONCLUSIONS: The results suggest that the underlying assumptions of the QALY calculation model are not in line with behavior from a real life population, implying that the QALY outcome measure might not be a valid measure for supporting health decision making in Europe. The findings of this first European experimental survey testing the validity of the QALYoutcome measure should be considered by European member states before recommending such approach in HTA guidelines.