Pre(sub)clinical Cushing's disease is a recently described entity defined by the autonomous secretion of ACTH and the absence of a cushingoid appearance. We screened 77 hospitalized patients with diabetes mellitus for subclinical hypercortisolism and detected pre(sub)clinical Cushing's disease in 2 (2.6%) of them. In both patients, transsphenoidal surgery was performed and a microadenoma was removed. Their metabolic clearance rate of glucose measured by a glucose clamp study, an index of insulin sensitivity, significantly improved after surgery. Our results indicate that screening for subclinical hypercortisolism in diabetic patients might be useful, as surgery improves glucose tolerance and insulin sensitivity.