Alzheimer's disease (AD) and normal pressure hydrocephalus (NPH) are common forms of dementia in the elderly. Recent findings have suggested an involvement of brain-derived neurotrophic factor (BDNF) in the pathogenesis of AD. BDNF is an endogenous protein involved in the maintenance of neuronal function, synaptic plasticity and structural integrity in the adult brain. BDNF serum and cerebrospinal fluid (CSF) concentrations were assessed by a sensitive ELISA in 27 AD patients in comparison to 9 NPH patients and 28 age-matched healthy controls (10 CSF samples). We found a significant decrease of BDNF serum concentration in AD (18.6ng/ml) and NPH patients (18.1ng/ml) as compared to healthy controls (21.3ng/ml; p=0.041/p=0.017). BDNF serum concentrations did not correlate with CSF levels, age or MMSE scores both in AD and NPH patients. In unconcentrated CSF samples, BDNF could be detected in AD patients in 8/27 cases (29.6%; mean of 4.6pg/ml), in NPH patients in 1/9 cases (11.1%; mean of 6.4pg/ml) and in the control subjects in 5/10 cases (50%; mean of 1.6pg/ml) with no significant differences as regards mean concentration and frequency of detectable BDNF in CSF. The decrease of BDNF serum levels in AD and NPH may reflect a lack of trophic support and thus contribute to progressive degeneration in both diseases. In contrast to serum, CSF seems to be no useful source to determine BDNF in AD or NPH because of too low concentrations. Further examinations have to follow to elucidate the potential sources and the meaning of reduced BDNF levels in the blood in AD and NPH.