This paper presents a discussion on the assessment of mental competence of patients and relatives in the health care setting Their decision-making ability is explored with particular reference to the giving of consent for nursing intervention and medical treatment It is suggested that, in addition to obvious psychological or neurological disorders, decision-making ability will be influenced by psychological stress and the ability to communicate effectively It highlights the ambiguity of mental competence assessment and concludes that such assessments will, to a greater or lesser extent, be subject to value-laden judgement