We measured serum alpha 1-antichymotrypsin (ACT) levels in patients with Alzheimer's disease (AD), Parkinson's disease (PD), Multiple System Atrophy (MSA) and age-matched controls to evaluate whether serum ACT levels in AD patients were elevated and whether ACT levels in PD patients with dementia differed from those in PD or AD. None of the patient groups displayed an increase in ACT levels. We conclude that serum ACT is not useful as a marker, nor in AD nor in dementia in PD.