["Immune activation and inflammation represent critical factors in the pathogenesis of Alzheimer's disease (AD) and are associated with increased blood concentrations of markers like neopterin and the kynurenine to tryptophan ratio (Kyn/Trp). In chronic inflammatory conditions, also increased serum phenylalanine concentrations and phenylalanine to tyrosine ratios (Phe/Tyr) were reported and could relate to neuropsychiatric symptoms.", 'To examine serum phenylalanine and tyrosine concentrations in patients with AD and to compare results to neopterin and Kyn/Trp levels.', 'Serum was collected from 43 patients with AD and concentrations of phenylalanine and tyrosine as well as neopterin, tryptophan and kynurenine were measured and Phe/Tyr and Kyn/Trp were calculated.', 'A subgroup of AD patients presented with increased phenylalanine and Phe/Tyr concentrations, phenylalanine levels correlated with neopterin (rs=0.329), kynurenine (rs=0.352) and Kyn/Trp levels (rs=0.288; all p<0.05). There was no significant relationship between phenylalanine metabolism and cognitive ability test scores mini-mental state examination and clock drawing test.'] Is immune activation in patients with Alzheimer 's disease associated with high serum phenylalanine concentrations?