Because cancer is a life-threatening illness, its impact on the patient's emotional well-being, such as suicidal thoughts, has become a significant problem in public health as well as in clinical oncology. Factors such as the pain and hopelessness are suggested as making cancer patients more vulnerable to suicide. On the other hand, euthanasia and physician-assisted suicide are now important medical and social issues all over the world. However, little is known about the relationship between the characteristics of cancer patients and suicidal thoughts. The present study investigated the characteristics of patients who were referred to the Psychiatry Division, National Cancer Center Hospital East, due to risk of suicide or suicide attempts. Fourteen patients were referred, representing 3.9% of all consultations. Most of these patients suffered from advanced cancer and poor physical functioning. The most frequent psychiatric diagnosis was mood disorder (57%), and the next was delirium (29%). In patients with mood disorders (8 cases), suicidal thoughts disappeared after psychiatric treatment in 5 cases, but not in 3 cases. Those three patients survived a significantly shorter time than the others after psychiatric consultation. These empirical data might indicate that most suicidal thoughts experienced by cancer patients are not rational, and a careful evaluation, including psychiatric assessment, should be conducted in such patients.