Diabetes mellitus is characterized by recurrent metabolic abnormalities which postmortem studies suggest might be associated with degenerative changes in the central nervous system. Acute hypoglycemia does indeed lead to cognitive impairment, whereas acute hyperglycemia in the absence of ketoacidosis or hyperosmolarity does not. Insulin-dependent diabetes mellitus is associated with cognitive deficits that tend to be relatively slight, inconsistent between different studies, and unrelated to clinical indicators; they can be ascribed as plausibly to psychogenic factors as to degenerative disease. In contrast, cognitive impairment in noninsulin-dependent diabetes mellitus is more conspicuous in tests of learning and memory, consistently associated with a patient's level of glycemic control, and more plausibly to be ascribed to structural neuropathology. Nevertheless, in both cases the deficits in question are unlikely to interfere significantly with patients' everyday functioning.