The differential diagnosis between a psychotic reaction and a psychotic reaction and mental deficiency is important because of implications regarding prognosis, treatment, and compensation benefits. In 21 patients who had been given the diagnosis of psychotic reaction and mental deficiency, re-examinations have revealed that mental deficiency could be definitely excluded in 17 or 81% of the cases. In view of these results, it would seem desirable that all patients who have previously been given a diagnosis of psychosis and mental deficiency be re-evaluated through a careful review of the clinical case history. Psychometric information should be considered valid only if obtained by a qualified examiner using an appropriate test.In 7 of 28 patients having the diagnoses of psychotic reaction and mental deficiency, no final conclusion could be reached as to the existence of mental deficiency because of inadequate or unreliable history and inability to test these patients properly. Periodic reexaminations and reviews of such patients are urged to reduce misdiagnoses of this type to a minimum.Abstract Teaser