Recent data indicate that a neuroimmune reaction might be responsible in part for neuronal death and cognitive deterioration in senile dementia. The potential involvement of brain histamine (HA) and interleukin-1 (IL-1) in this process has been previously documented. We have studied the concentration of serum HA in patients with Alzheimer's disease (AD) or multi-infarct dementia (MID) and in age-matched control subjects. Serum HA levels were significantly higher in AD (10.935 +/- 5.692 nM) and MID (8.521 +/- 3.44 nM) than in controls (5.533 +/- 2.567 nM) and correlated with mental performance as evaluated with the Mini-Mental State Examination (MMSE) (r = +0.493, p < 0.009). No correlation was found with cardiovascular parameters, cerebrovascular risk factors or age. Hyperactivation of the histaminergic system in AD at central and peripheral levels might reflect a neuroimmune reaction to brain tissue damage, a neurotrophic response, and/or a reactive process to regulate the IL-1 induced amyloid precursor protein (APP) overproduction.