Introduction. The study of changes in family law in Russia in the 1917-1920s under the influence of biomedical factors is becoming particularly relevant today due to the rapidly developing technologies in the modern world and the special significance of the institutions of family, motherhood, fatherhood and childhood. Consideration from two sides: medical and legal issues of marriage between blood relatives, persons incapacitated, pre-marital medical examination, problems of establishing paternity, etc., will allow a broader and objective assessment of the importance of human health protection and timely regulation of family legislation. Purpose. The purpose of the study is to study the role of biomedical factors in the formation of Russian family law in the 1917-1920s. Methodology. When writing the work, general philosophical methods (dialectical-materialistic), general scientific methods (analysis, synthesis, logical and historical, comparisons, etc.), private scientific (formal legal, interpretation of law, etc.) were used. Results. The analysis of the first Family Code - the Code of Laws on Acts of Civil Status, Marriage, Family and Guardianship Law, adopted on September 16, 1918, the second codified act regulating family relations - the Code of Laws on Marriage, Family and Guardianship, put into effect on January 1, 1927, as well as adopted by the Soviet state in 1917-1920-e. decrees and instructions supplementing and clarifying the basics of family and marriage legislation. The study of manuals, manuals, reference books addressed to doctors during forensic medical examinations, as well as the study of judicial practice, made it possible to identify the most pressing problems encountered when considering paternity cases. Conclusion. The authors conclude that the transformation of post-revolutionary family legislation, the expansion of medical grounds, the clarification of medical diseases that are an obstacle to marriage, testified to the influence of new views on the tasks of the state in the field of family law. These measures were aimed at supporting the family by the state, preventing the transmission of hereditary diseases, and, ultimately, preserving the health of future offspring. The legislative equation in the rights of children born in marriage and out of wedlock, the imposition of the duty on the father of the child to provide for the family, to take part in the upbringing of children, in practice led to the need to establish paternity. When considering this category of cases, the courts, protecting the interests of mothers and children, made decisions based on the available evidence, which often contradicted the principles of objectivity and admissibility. In this regard, the conclusions of forensic medical experts have become particularly important.