The increasing incidence of chronic diseases is becoming a heavy burden for both public and private healthcare sectors. Many industrial and academic efforts are trying to alleviate this burden using various clinical solutions. Establishment and execution of prevention plans and subjects' monitoring are among the promising solutions. In keeping with these efforts, we describe in this paper our proposed architecture, for health monitoring of patients with chronic diseases. The architecture relies on the service Oriented Architecture (SOA) and cloud computing for the implementation and integration of services from various stakeholders. The monitoring scheme allows to proactively detect risks of disease' aggravation and dynamically generates and customizes prevention plans according to the patient's health profile and context. A prototype of our system is under development and preliminary data have been collected and analysed.