The contribution of immunological factors in the etiopathogenesis of Alzheimer's disease (AD) is increasingly noted. Apart from cerebral immunological findings, peripheral changes of the immune systems have been reported including lymphocyte function and subset distribution. As data still remain inconsistent, we investigated a sample of 43 patients with AD and of 34 healthy age-matched controls. Distribution of the T-, B- and NK cell subsets was determined by flow cytometry (FACS). We found a significant decrease of CD3(+) lymphocytes as well as of CD19(+) lymphocytes. A slight increase of the CD4(+) and a decrease of the CD8(+) subpopulation could be observed, without significant change of the CD4(+)/CD8(+) ratio. CD16(+)56(+) cells were not altered. Our findings of decreased T- and B-Cell numbers in AD sustain the hypothesis of a general decline of immune activity in AD. A putative association with premature immunosenescence in AD and possible pathogenetic implications are discussed.