The peripheral and central nervous system are harbouring herpes simplex virus type 1 (HSV-1) and this virus has been proposed to be implicated in the aetiology of Alzheimer's disease (AD). We tested whether the HSV-1 genome is found indeed in the brain of controls, patients with AD and Down syndrome (DS) and whether HSV-1 infectious proteins in brain were induced. Moreover, we tested whether interleukin (IL)-6, a marker for neuroinflammation, is found in brains of AD and DS. HSV-1 glycoprotein D gene, as well as viral phosphoprotein and glycoprotein were detected in all brain samples. IL-6 was detectable in seven out of the eight AD and all of the eight DS patients, but only three out of ten controls in the frontal cortex. IL-6 in cerebellum was detectable in all AD and DS patients, but only three out of nine controls. In conclusion, we propose that the detection of HSV-1 genome and HSV-1 inducible protein IL-6 not only shows the presence in human brain, but may indicate a role for HSV-1 in the process of neuroinflammation and apoptosis, known to occur in both neurodegenerative disorders, AD and DS.