Several cases which have been considered by the courts in recent years have highlighted the legal dilemmas facing doctors whose decisions result in the ending of a patient's life. This paper considers the case of Dr Cox, who was convicted of attempting to murder one of his patients, and explores the roles of motive, diminished responsibility and consent in cases of "mercy killing". The Cox decision is compared to that of Tony Bland and Janet Johnstone, in which the patients were in a persistent vegetative state. In all three cases, the doctors believed that their patients' quality of life was so poor that their continued existence was of no benefit to them, and decided that their lives should not be unduly prolonged, yet the doctor who was prosecuted was the one whose dying patient had requested that her death be hastened. The paper examines the law's seemingly contradictory approaches to such cases.