Brain tumor is found at necropsy in from 3.5 per cent, 6.7 per cent to 13.5 per cent in state hospital material.Brain tumor may occur in a patient with a major psychosis in which instance it is not necessarily the cause of psychotic symptoms but will undoubtedly aggravate them. Here the problem of the psychiatrist is to recognize the symptoms of a progressive neurologic disorder running concomitant with the symptoms of the major psychosis. An illustrative case is presented.Far more important are those cases of brain tumor in which the tumor is the cause of the presenting mental symptoms. Meningiomas arising from the olfactory groove are the most important from a psychiatric standpoint because they are benign and operable. If Foster Kennedy's syndrome is well understood and evaluated these tumors can be recognized much earlier and more frequently than they now are.The brain tumor problem in psychiatry is almost as important as is the problem of syphilis, and it furnishes one of several good reasons for not making too great a separation between neurology and psychiatry.Such diagnoses as Alzheimer's disease, Pick's disease, arteriosclerotic dementia, encephalitis, and Parkinson's disease should be regarded with a high degree of suspicion until tumor has been ruled out. The burden of proof certainly rests on the psychiatric diagnostician. We as psychiatrists should become more "brain tumor conscious" and air injection should be employed more frequently than it is employed in psychiatric clinics. When it is remembered that the meningioma, a benign operable tumor is the most common tumor producing mental symptoms there is all the more reason for improving our diagnostic skill in recognizing tumor as a cause of mental symptoms.Illustrative cases are presented together with necropsy, operative and encephalographic studies in patients in whom one or more examiners failed to recognize tumor as the cause of the psychotic symptoms.