We assayed cytochrome oxidase and other electron transport chain activities in platelet mitochondria isolated from patients with Alzheimer's disease (AD). Five of 6 patients had striking reductions of platelet cytochrome oxidase activity (patient mean, 83.72 +/- 82.99 nmol/min/mg; control mean, 167.14 +/- 36.21 nmol/min/mg; n = 8). Other electron transport chain catalytic activities were not significantly different than control values. AD may be a systemic illness, a primary defect in cytochrome oxidase may be pathogenically important in its production, and the mitochondrial genes encoding cytochrome oxidase subunits may be important in producing the defect.