Evidence-based medicine relies on the execution of clinical practice guidelines and protocols. A great deal of effort has been invested in the development of tools which can automate the representation and execution of the recommendations contained within such guidelines, by creating Computer Interpretable Guideline Models (CIGMs). Context-based task ontologies (CTOs), based on standard terminology systems like UMLS, form one of the core components of such models. We have created DAML+OIL-based CTOs for the tasks referred to in the WHO guideline for hypertension management, drawing comparisons also with other, related guidelines. The advantages of CTOs include: contextualization of ontologies, tailoring of ontologies to specific aspects of the phenomena of interest, division of the complex tasks involved in creating ontologies into different levels, and provision of a methodology by means of which the task recommendations contained within guidelines can be integrated into the clinical practices of a health care set-up.