THE facts set forth in this paper are based upon a study of 19 cases of obesity and 19 control cases all of which came to autopsy in the Royal Infirmary, Edinburgh. The former were ordinary fat people, most of them elderly women, with a definite and in some cases a gross excess of fat, mainly subcutaneous and mesenteric in distribution. The subjects chosen as controls were individuals of the lean type with little or no subcutaneous fat, but no control case was emaciated. In all cases the urine was sugar-free during their residence in hospital. The subjects of both series died from a variety of causes, eg chronic interstitial nephritis, cerebral abscess, chronic endocarditis, lobar pneumonia, etc. Since no difference exists between the two series so far as causes of death are concerned it has been deemed unnecessary to give details. Finally, it should be mentioned that the research was strictly limited to a study of the islands of Langerhans and that in no case was attention paid to any of the other glands of internal secretion.