In aging humans and rodents, inter-individual differences in cognitive function have been ascribed to variations in long-term glucocorticoid exposure. 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) regenerates the active glucocorticoid cortisol from circulating inert cortisone, thus amplifying intracellular glucocorticoid levels in some tissues. We show that 11β-HSD1, but not 11β-HSD2, mRNA is expressed in the human hippocampus, frontal cortex, and cerebellum. In two randomized, double-blind, placebo-controlled crossover studies, administration of the 11β-HSD inhibitor carbenoxolone (100 mg three times per day) improved verbal fluency (P < 0.01) after 4 weeks in 10 healthy elderly men (aged 55-75 y) and improved verbal memory (P < 0.01) after 6 weeks in 12 patients with type 2 diabetes (52-70 y). Although carbenoxolone has been reported to enhance hepatic insulin sensitivity in short-term studies, there were no changes in glycemic control or serum lipid profile, nor was plasma cortisol altered. 11β-HSD1 inhibition may be a new approach to prevent/ameliorate cognitive decline.