The aim of the article is scientific conceptualization of the problem of mental derangement as a mandatory medical criterion of limited sanity according to criminal legislation by experience of Ukraine and some foreign countries and development of new approaches to such status assessment. Empirical study base of the problem consists of judgments of conviction taken from Unified State Register of Court Rulings of Ukraine and carried out on persons who commit crimes in the state of limited sanity from March 1st, 2014, till August 1st, 2020. Results of some foreign researchers' studies of the problem in question have been used as well. A combination of general and special scientific methods (comparative method, system structural method, method of statistical analysis, method of legal phenomenon system analysis etc.) has been applied to reach the aim in view. In conclusion: medical criterion of limited sanity ("mental derangement") used by lawmaker does not allow defining clearly types of illnesses, represent all possible types of psychiatric pathology and also causes complications in law enforcement practice. It has been proved that increase of quantity of people having mental derangements and quantity of crimes committed by such people shows that non-application of compulsory measures of medical care, correlational programs leads to repeated crimes commitment. Taking into account absence of duration of compulsory measures of medical care application, it has been suggested to stipulate this by Ukrainian legislation and to develop classification of mental derangements and criteria of their division into severe and non-severe.