Hyperparathyroidism was diagnosed in a 67-year-old diabetic man treated for 20 years with isophane insulin suspension, 40 to 45 units/day. It was also diagnosed in a 64-year-old diabetic with severe retinopathy and vascular disease, who was not dependent on insulin. In the first case, removal of a parathyroid adenoma resulted in frequent hypoglycemic attacks, which led to a reduction of the administration of insulin isophane suspension to 20 units/day. In the second case, there was a notable improvement in the glucose tolerance testing that followed surgery, accompanied by a decrease in total plasma insulin response from 17,838 to 5,605 units, by planimetry. These observations suggest that hyperparathyroidism worsens coexisting diabetes mellitus and that one must be aware of increased insulin sensitivity and the possibility of severe hypoglycemia in cases that require insulin after surgical correction of the hypercalcemic state.