Patients with advanced cancer may have adaptive disorders that respond to an inconvenient situation in many dimensions of life. Defensive mechanisms of the personality allow them to reduce the intensity of unpleasant emotions without changing the situation that causes them. Through this, they can help to cope with stress and tension caused by the crisis situation. When applied excessively or inadequately to the situation, they may hinder the functioning of the oncological patient both by the inability to assess their own condition, the lack of communication with the treating physician, and indecision in the choice of an effective treatment method. This paper presents a case of a patient with advanced ovarian cancer, in whom the perpetuation of the defence mechanisms of the personality had an adverse impact on the course of the disease, which was a radical denial regarding the diagnosis and consequences of the disease and, therefore, discontinuation of the proposed treatment.