The decision making capacity of patients will acquire special relevance with the introduction of informed consent in clinical practice and the new normative that will appear in the future when the bills about health services are approved in the Congress. This paper reviews the concept of decision making capacity in the context of health care and its legal background in Chile. The main problems that arise from the comprehension, use and assessment of decision making capacity, are analyzed. The assessment of this capacity is, in most cases, subjective since there are no criteria, protocols or standards to be used. In courts of justice, the «exercise capacity» assessment is requested to experts. These specialized physicians will evaluate the mental health of the subject and its implications in the capacity to make decisions. In practice, it is difficult to integrate the concept of capacity from the law or health care perspective. It is concluded that the elaboration of criteria, standards and procedures to evaluate the decision capacity of patients, is mandatory (Rev Méd Chile 2004; 132: 1243-8)