We studied respiratory chain enzyme activities in lymphocyte mitochondria from 33 patients with Alzheimer's disease (AD) and from 30 age- and sex-matched healthy controls. The respiratory chain enzyme activities did not differ significantly between patients and controls. No patient showed any value for respiratory chain enzyme levels below normal range. Values for activities of complexes in the AD group did not correlate with age at onset or duration of the disease. Our finding of normal mitochondrial function in lymphocyte mitochondria suggests that this tissue cannot be used to demonstrate the involvement of oxidative phosphorylation in AD and, thus, to develop a diagnostic test for AD.