Objectives Implicit factors can be defined as any criteria playing a role in the HTA deliberative process but that are not being part of the HTA framework. To date, only very few studies have explored the influence of implicit factors on this process. This survey of HTA experts in 5 European countries aimed to analyze the influence of implicit factors on the HTA deliberative process. Methods Semi-structured interviews with HTA experts (n = 20) from 5 European countries (Spain, France, Italy, Germany and United Kingdom) were conducted in February-May 2021. The main topics of the interviews were (1) the HTA deliberative process, (2) the degree of influence ion HTA deliberative process of a set of factors that were previously identified in a systematic literature review performed by the authors and (3) recommendations to improve the deliberative process. Results 18/20 HTA experts concurred that implicit factors play a role in the process. Only 2 German experts considered that they have a low influence on HTA deliberative process. The implicit factors scored with the highest influence in the process were (1) burden of disease, (2) unmet need and (3) professional experience of the members participating in the HTA deliberative process. To improve the deliberative process, 12 experts suggested to increase the external stakeholder perspective in the process (i.e. patients, pharmaceutical industry, general public), 6 stressed the importance of revealing implicit factors and 5 recommended to develop a methodology to quantify the influence of these factors. Conclusions Our survey indicates a need for increasing external involvement in HTA deliberative process and for developing a methodology to unmask implicit factors that influence this process. This may be achieved by either updating the current deliberative process frameworks to include these factors or by developing new methods that address them.