In Japan, cancer has been the leading cause of death since 1981 and is the most common and representative life-threatening disease. Cancer treatment has markedly progressed, and so the development of cancer is now not necessarily a death sentence. Nevertheless, approximately half of cancer patients actually die as a result of the disease even today. Cancer causes marked suffering of patients.
 Patients with advanced and/or terminal cancer suffer from various physical symptoms and are forced to face a continuous decline in their physical function, and previous studies reported that psychological distress is frequently observed in cancer patients, especially in advanced and/or terminally ill patients. Several types of psychological defense mechanism, especially denial, are frequently observed in these patients. There are multiple levels of denial. For example, a patient with true denial denies the presence of cancer itself, although true denial is quite rare. A patient with moderate denial is likely to deny an association between distressing symptoms and disease, while one with mild denial denies the life-threatening nature of disease. Denial is a psychological defense mechanism that often helps advanced cancer patients to manage their anxiety and depression. It allows such patients to continue living while facing the painfulness of their situation. In this situation, it is important for therapists to note the patient's denial. Simply being aware of the denial can, in itself, be supportive for the patient. However, when denial does not work adaptively, such as it prohibiting adherence to necessary treatment and not being useful for ameliorating psychological distress and improving the quality of life, medical staff may consider confronting a patient with the reality of the situation carefully. This paper discusses the relevance of acknowledging denial and caring for patients with denial from the point of view of psycho-oncology.
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