Introduction. The formation and development of modern evidence-based medicine is not possible without clinical studies with the participation of humans as subjects. The methodology of such experiments is based on careful selection of subjects based on the so-called eligibility criteria – parameters which form the optimal patient group for studying the therapeutic properties of a particular drug based on the science community’s expectations. The article considers some bioethical aspects of the practical problems of formulating such criteria, and also it examines the question of the possibility of proper application of scientific data obtained in the framework of clinical studies. Theoretical analysis. Based on the example of HIV-positive patients, it is shown that excessive formalization of the process of determining eligibility criteria leads to the formation of fundamental differences between participants in clinical trials and patients in routine medical practice, which put at risk the ability to properly extrapolate the results of such studies outside the experimental field and realization of one of the basic bioethical principles of clinical trials: to be useful for society. Conclusion. The article substantiates the need to revise the methodological and bioethical foundations of the process of formulating eligibility criteria, taking into account the risks faced by patients of experimental therapy and patients in real clinical practice.