Ambulatory health care in developing countries is often provided by medical or paramedical personnel commonly called village health workers. Medical problems within their realm of competence include diarrhea, infestation with worms, diseases affecting the eyes and the skin, and several kinds of infectious diseases. Also, assessment of malnutrition is among the tasks village health workers have to perform. A knowledge-based system is designed and implemented to provide decision support for those tasks. Expanded decision networks are used to represent the diagnostic processes for diarrhea and infestation with worms. The diagnostic procedures for diseases affecting the skin and the eyes are covered with a rule-based approach using two certainty factors. Treatment schemes and procedural knowledge about drug prescription are represented in a frame structure. The hybrid knowledge engineering tool VIE-KET is used to construct a modular consultation system that provides entry points for diagnosis, therapy, and drug prescription. Different aspects of the design and implementation of the system are discussed, and two sample consultation sessions are presented.