Introduction (relevance): Since 2019, an order of the Russian Ministry of Health has established a requirement for medical organizations to compulsorily record adverse events that pose a threat or have resulted in harm to the life and health of patients or medical workers, which is designed to facilitate the accumulation of negative experience to identify patterns of adverse events and subsequently identify hidden risks. But despite the existence of this requirement, the process of identifying and recording adverse events in medical organizations raises many questions, including which events are unwanted and how to systematize them for further analysis and development of corrective and preventive measures. Objective: systematize unwanted events that are possible during medical activities. Results: classification of undesirable events is a systematic list of undesirable events, grouped according to homogeneous characteristics: according to the level of implementation of the undesirable event and the degree of influence on the person (patient or medical worker); according to the circumstances of their occurrence; due to occurrence. To simplify statistical processing and subsequent analysis, it is proposed to code unwanted events. Coding of unwanted events will make it possible to depersonalize information about where, when and with whom an unwanted event occurred, leaving only the information that is necessary for conducting a systemic analysis, comparison across groups, organizations and regions, as well as developing a system of measures to prevent them. Conclusions: classification of undesirable events possible in various areas of activity of medical organizations provides a more complete identification of them, which, in turn, contributes to their systematization, analysis, identification of causes and development of measures for elimination and prevention.