Available evidence indicates that the internet is becoming the main source for delivering information on the prevention, control and treatment of HIV/AIDS. This paper presents a further step in the use of the internet to meet the new challenge of managing HIV as a chronic illness. It describes a work carried out under the SEAHORSE EU project to build and test a Web-based self-monitoring system for HIV/AIDS patient care. The user interface has been carefully designed to provide a high-level of interaction and therefore improve some of the current limitations of Web applications. The system comprises three modules: (1) a patient self-monitoring personal diary, to create a follow-up patient record; (2) a data analysis and visualisation tool; and (3) a section to allow patients to ask for advising and remote doctor support. One of the crucial system design issues has been the system security and users anonymity features required in this clinical domain. A feasibility pilot has been carried out to test the system in three organisations involved in the research project: the Immune Development Trust (London), the Lambeth, Southwark and Lewisham Health Authority (London) and Apoyo Positivo in Madrid. The preliminary results of the system evaluation show the potential validity and usefulness of the tool for helping people living with HIV/AIDS to promote and manage their health and for providing health professionals with new means for tele-monitoring and tele-caring patients.