The potential of impairments in oxidative/energy metabolism to cause diseases of the brain had been proposed even before the major pathways of oxidative/energy metabolism were described. Deficiencies associated with disease are known in all the pathways of oxidative/energy metabolism and are associated with some of the most common disorders of the nervous system, including Alzheimer's disease (AD) and Parkinson's disease. A common mechanism in these conditions appears to be a downward mitochondrial spiral, involving abnormalities in energy metabolism, calcium metabolism, and free radicals (reactive oxygen and nitrogen species). In AD, the spiral appears to interact with abnormalities in the metabolism of the Alzheimer amyloid precursor protein (APP) and its Aβ fragment. Several lines of evidence indicate that the mitochondrial spiral may be a proximate cause of the clinical disabilities in AD. Decreases in cerebral metabolic rate (CMR) characteristically occur in AD and in other dementias. Inducing decreases in CMR leads to clinical disabilities characteristically associated with AD and with analogous problems in experimental animals. Treatments directed toward normalizing CMR appear to help at least some patients. Further studies of this possibility and of treatments designed to ameliorate the mitochondrial spiral may prove useful for treating AD and perhaps some other dementing disorders. © 2001 Wiley-Liss, Inc.