In Japan, as in some other countries, not all cancer patients are informed of their diagnosis and expected prognosis. However, we do not know enough about the relationship between the kind of information given to cancer patients and their mental state. The following is a study that examines this relationship. The subjects were 81 adult cancer inpatients referred to psychiatrists. Those with mental disorders before admission or with delirium were excluded. The subjects were classified into three groups according to the kind of information given to them: 28 were not informed of a diagnosis of cancer, 36 were informed of a diagnosis of cancer, and 17 were informed of both a diagnosis of cancer and a poor prognosis. The distributions of DSM-IV Axis I diagnoses were nearly equal among the three groups of patients. In each group, more than 90% of patients were diagnosed as having either major depressive disorder or adjustment disorders. In the results of HAM-D, HAM-A, and a question about irritability, the mean total HAM-A scores and mean scores for the factors of Suicide, Behavior at Interview, and Irritability, differed significantly among the three groups. They were highest in patients who were not informed of a diagnosis of cancer. The authors discuss the nature of relationship between the disclosure of information and the mental state in cancer patients.