Global cerebral blood flow, oxidative brain metabolism, and the cerebral arteriovenous differences of amino acids and ammonia were studied in 20 clinically diagnosed patients with early-onset dementia of Alzheimer type (DAT). Eleven healthy age-matched subjects and 15 healthy young volunteers served as controls. The most prominent abnormality in patients with early-onset DAT was a 44% reduction in the cerebral metabolic rate of glucose and a fourfold increase of lactate production, whereas cerebral blood flow and the cerebral metabolic rate of oxygen were found not to be altered. The cerebral amino-N balance substantially changed in patients with early-onset DAT, showing a massive loss of amino acids and ammonia from the brain, which was indicative of excess protein catabolism due to cell degeneration in the acutely diseased brain. The abnormality found in glucose metabolism may suggest a perturbed control of glycolytic breakdown of glucose and its first oxidation step at the pyruvate dehydrogenase complex level, this thus being of pivotal significance in early-onset DAT.