The recent important developments in our knowledge of the pathogenesis and treatment of diabetes insipidus are intensely interesting to the medical profession. The Mayo Clinic has afforded exceptional opportunities for the clinical study of this affection, thirty-seven cases having been observed during the last four years. This report comprises the results of statistical and clinical analyses of all the cases of diabetes insipidus observed in the division of medicine of this clinic since 1910 (fifty-six in number), and a survey of the laboratory findings and results of treatment in thirty-seven cases observed during the last four years. As my own connection with the Mayo Clinic dates from 1920, the cases will be considered in two groups: Series 1, composed of nineteen cases observed between Jan. 1, 1910, and April 1, 1920, and Series 2, composed of thirtyseven cases observed between April 1, 1920, and April 1, 1924.1 INCIDENCE The