Several hereditary and other chronic diseases necessitate continuous and complicated health care procedures, typically offered in different, often distant, health care units. Inevitably, the medical records of patients suffering from such diseases become complex, grow in size very fast and are scattered all over the units involved in the care process, hindering communication of information between health care professionals. Web-based electronic medical records have been recently proposed as the solution to the above problem, facilitating the interconnection of the health care units in the sense that health care professionals can now access the complete medical record of the patient, even if it is distributed in several remote units. However, by allowing users to access information from virtually anywhere, the universe of ineligible people who may attempt to harm the system is dramatically expanded, thus severely complicating the design and implementation of a secure environment. This paper presents a security architecture that has been mainly designed for providing authentication and authorization services in web-based distributed systems. The architecture has been based on a role-based access scheme and on the implementation of an intelligent security agent per site (i.e. health care unit). This intelligent security agent: (a). authenticates the users, local or remote, that can access the local resources; (b). assigns, through temporary certificates, access privileges to the authenticated users in accordance to their role; and (c). communicates to other sites (through the respective security agents) information about the local users that may need to access information stored in other sites, as well as about local resources that can be accessed remotely.