The purpose of this study was to determine cerebral myo-inositol (ml) in adults with Down syndrome (DS), and to trace the chronobiology of DS to Alzheimer disease (AD). AD has characteristic neuropathology of neurofibrillary plaques and tangles; indirect evidence links this to earlier deposition of β-amyloid. Elevated ml, which distinguishes AD from other common dementias, is also elevated in 23 young patients who have DS without dementia. In one patient who has DS with dementia, ml is elevated and N-acetylaspartate (NAA) decreased. The similarity to AD is striking and may suggest a progressive neurochemical disorder in which elevation of ml precede loss of NAA in both AD and DS.