Basic and far-reaching innovations in the structure of medical knowledge were made in Paris in the early part of the nineteenth century. In order to explore the social basis of these innovations, attention is focused on the development of hospitals in Paris, and the way in which this development was associated with the emergence of the doctor as the dominant figure in the doctor-patient relationship. This type of relationship is contrasted with the structure of practitioner-client relationships in the eighteenth century, which were characterized by a structure of client control. Some of the ways in which the dominance of the doctor within the hospital situation facilitated innovation in medical knowledge are examined.