CONTEXT: The etiology of idiopathic intracranial hypertension (IIH) is unknown. We hypothesized that obesity and elevated intracranial pressure may be linked through increased 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) activity. OBJECTIVE: The aim was to characterize 11β-HSD1 in human cerebrospinal fluid (CSF) secretory [choroid plexus (CP)] and drainage [arachnoid granulation tissue (AGT)] structures, and to evaluate 11β-HSD1 activity after therapeutic weight loss in IIH. DESIGN AND SETTING: We conducted in vitro analysis of CP and AGT and a prospective in vivo cohort study set in two tertiary care centers. PATIENTS OR OTHER PARTICIPANTS: Twenty-five obese adult female patients with active IIH were studied, and 22 completed the study. INTERVENTION: Fasted serum, CSF, and 24-h urine samples were collected at baseline, after 3-month observation, and after a 3-month diet. MAIN OUTCOME MEASURES: Changes in urine, serum, and CSF glucocorticoids (measured by gas chromatography/mass spectrometry and liquid chromatography/tandem mass spectrometry) after weight loss were measured. RESULTS: 11β-HSD1 and key elements of the glucocorticoid signaling pathway were expressed in CP and AGT. After weight loss (14.2±7.8 kg; P