The aim of the present study was the quantitation of total tau protein (tau(T)), tau phosphorylated at threonine 181 (tau(P-181)) and beta-amyloid(1-42) (Abeta42) in the cerebrospinal fluid (CSF) of patients with idiopathic normal pressure hydrocephalus (iNPH), Alzheimer's disease (AD) and controls. Double sandwich ELISAs (Innogenetics) were used for the measurements. Total tau was significantly increased in iNPH and highly increased in AD as compared with the control group, whilst Abeta42 was decreased in both diseases. CSF tau(P-181) levels were significantly increased only in AD, but not in iNPH as compared with the controls. A cut-off level for tau(T) at 300 pg/ml, successfully discriminated AD from normal aging with a 95.8% specificity and 91% sensitivity; whilst the tau(P-181)/tau(T) ratio (cut-off value 0.169) was more specific (100%) but less sensitive (92.5%). For the discrimination of iNPH from AD tau(T) achieved low specificity (77.8%) but high sensitivity (92.5%), whilst tau(P-181) (cut-off value 47.4) was both sensitive and specific (88.7% and 86.7% respectively) for the discrimination of these disorders. The present study, despite being clinical, supports the notion that CSF tau(P-181) alone or in combination with tau(T) may be a useful marker in the discrimination of iNPH from AD.