The material on which this communication is based was obtained during a survey of all the patients in Severalls Mental Hospital, one of the objects of which was to obtain information about the incidence of the various types of mentally disordered and mentally defective patients. An account of this survey was published in the Journal of Neurology and Psychopathology for January, 1936. For the purpose of the present more detailed analysis of the results relating to manic-depressive insanity and mental deficiency, mental ability has been graded as (1) feeble-mindedness, including all patients certifiably defective, (2) dullness, i.e., subnormal ability without certifiable defect, (3) normal.