This paper introduces a conceptual framework which can be used to study the sustainability of health services in developing countries. A health service is considered sustainable when operated by an organizational system with the long-term ability to mobilize and allocate sufficient resources for activities that meet individual or public health needs. The framework includes three clusters: (1) contextual factors, which outline the task and general environment of the services; (2) an activity profile, which describes the services delivered and the activities carried out to deliver them; and (3) organizational capacity, which shows the carrying ability (capability) of the organization in broad terms. In this framework, health care provision is seen as an open system model where five main factors determine how inputs are converted to outputs, linking them through feedback loops. These factors are aims, technology, structure, culture and process. The framework has proven useful in analyzing factors critical to sustainability, and in describing structures and processes both in basic public services and in private not-for-profit services. It should also be tested on more complex systems, such as national health care.