The paper will discuss the problem of post-sentence preventive detention of dangerous offenders, primarily from the perspective of the collision between the need to respect the personal liberty of individuals and the positive obligations of the state to protect public order and the rights of others. The ECtHR in the most recent case law accepted the use of post-sentence preventive detention provided that it is limited to persons of “unsound mind” and is executed in the therapeutic environment. However, in practice both these conditions are unclear: the ECtHR has never defined, even in outline, the term “unsoundness of mind”, while the involuntary therapy of offenders with non-psychotic disorders is ineffective. Consequently, the ECtHR’s jurisprudence does not give precise answer as to what are the limits of preventive detention. The paper will address this issue in the light of the contemporary human rights standards as well as the Polish experiences with post-sentence detention.