Canessa et al observed 17 people with untreated obstructive sleep apnea and 15 age-matched healthy controls, performing sleep studies, cognitive testing, and MRIs on all. At baseline, OSA patients had shrinkage in brain areas responsible for memory and executive function (hippocampus, posterior parietal cortex, and superior frontal gyrus, as quantitated by voxel based morphometry) and were neurocognitively impaired relative to controls. After 3 months of CPAP treatment, all the brain abnormalities resolved on MRI, and neuropsych testing was significantly improved in most domains. AJRCCM 2011;183:1419-1426.