Studies were made of glucose tolerance and other characteristics related to diabetes in fifty-one patients with hyperthyroidism. Glucose tolerance compatible with diabetes was present in 57 per cent of patients when toxic and 30 per cent on return to euthyroidism. Histories of family diabetes in both sexes and of heavy babies delivered by the women were obtained more frequently than expected in the general population. Reasons are given to suggest a genetic relation between hyperthyroidism and diabetes. DIABETES 14:740-44, November 1965.The prevalence of diabetes mellitus in clinical hyperthyroidism is not known with certainty. Estimates of 2.0 to 3.3 per cent have been given, figures little different from that expected for genetic diabetes in the general population. 1 Since this frequency seemed lower than that encountered on the Medical Service at the Cincinnati General Hospital, studies were made of glucose tolerance in hyperthyroid patients over a three-year period. In addition, observations were made of other traits related to genetic diabetes mellitus. METHODS(1) Patients. The sample for study included all patients in whom the diagnosis of hyperthyroidism was made by the Medical Service of the Cincinnati General Hospital from 12/1/60 to 11/30/63. There were fortyfour women and seven men. The mean age was fortythree years. The distributions of thyroid disease and ages are given in table 1.(2) Assessment of thyroid function. Hyperthyroidism was diagnosed by symptoms, signs, and appropriate tests. The mean weight loss claimed was twenty-five pounds or 16.5 per cent of pretoxic weight. The mean