I believe that I have demonstrated the independence of pure melancholia from manic-depressive illness by the clinical descriptions and the family histories. It is a unipolar disease, without manic phases. Clinical differentiation from manic-depressive illness is not possible in every case, but evidence from family histories has shown misdiagnosis to be rare. The clinical picture is characterized by depression, psychomotor inhibition, thought inhibition, indecision, feelings of insufficiency, and varying depressive ideas, not particularly obtrusive in any one direction. The danger of suicide is great. Somatic symptoms add to the psychic picture, as with the melancholia of manic-depressive illness. Comparative investigations, however, would be desirable.