This paper provides an analysis on the use of 15D and EQ-5D to measure health related quality of life. Measures like these are often used interchangeably in cost-effectiveness studies. However, it is unclear whether they measure the same level of health in the same patients. The empirical performance of the two multi-attribute utility instruments is tested in terms of feasibility, utility score, linear relationship and agreement by using a novel Norwegian data set. The paper also includes an analysis of how the instruments rank individuals in terms of health status, and their discriminatory power are tested. The results show that EQ-5D and 15D should not be used interchangeably in economic evaluations. EQ-5D is likely to give a more favourable cost utility ratio than 15D. The utility scores generated from the two instruments differ significantly different from each other, even though they correlate well. The instruments also rank individuals in terms of health status differently.