Abstract Amyloid β-protein, the major constituent of Alzheimer's disease (AD) brain amyloid deposits, is encoded by several alternatively spliced amyloid precursor protein (APP) mRNAs. The well-established association in Down's syndrome (DS) between APP overproduction and premature development of AD, as well as the recent demonstration of an increase in APP transcripts from lymphoblastoid cells of familial AD cases, suggest aberrant transcriptional regulation of some genes in AD. We assayed steady-state expression of the APP gene transcripts in peripheral blood mononuclear cells (PBMC) of AD and DS patients using quantitative polymerase chain reaction of reverse-transcribed mRNAs, and we compared their levels of PBMC APP expression with those of young and age-matched healthy controls. Results indicate APP mRNAs were of comparable abundance in PBMC obtained from 9 AD patients, 6 DS patients, 7 young controls and 12 age-matched controls. These data suggest regulation of APP mRNAs is normal in AD and DS PBMC.