Ethics has been identified as a key element in Health Technology Assessment (HTA) since its conception. However, ethical issues are still not frequently addressed explicitly in HTA. Several valuable reasons have been identified. The basis of the article is the claim that ethics is often not part of HTA for "epistemological reasons". Hence, the main aim of the contribution is to explore in more details and emphasize them by using the fact/value dichotomy. Our conclusion is that current HTA configuration is predominantly based on the comparison among objective and empirically testable "facts", whilst ethics is not empirically testable. In this sense, there is a sort of "epistemological gap", which can explain why it is so difficult to integrate ethics in HTA. We suggest that the epistemological differences among the various domains of HTA are addressed more explicitly.