The article is devoted to the contribution of the English physician T. Sydenham to the formation of modern Western medicine. Based on the analysis of his major works, the authors concluded The contribution of T. Sydenham (1624-1689) to the formation of modern Western medicine (on the occasion of the 390th anniversary of his birth) that T. Sydenham's main merit was not the application of an empirical approach to studying human diseases, but the development of two theoretical concepts in the 1660s to 1680s, which introduced new ideas about the causes of diseases and epidemics. Recognition of T. Sydenham`s ideas by the majority of the medical community led to a fundamental revision of the views on illnesses, to the emergence of qualitatively new approaches to the study, diagnosis, treatment and prevention of human diseases, by giving rise to a new subject of medical study, such as physical and social environmental factors in man along with his body in health and disease.