Hyperphosphorylated tau protein (P-tau) in CSF is a core biomarker candidate of Alzheimer's disease. Hyperphosphorylation of tau is thought to lead to neurofibrillary changes, a neuropathological hallmark of this type of dementia. Currently, the question is unresolved whether CSF levels of P-tau reflect neurofibrillary changes within the brain of a patient with the illness. Twenty-six patients were included with intra-vitam CSF as well as post-mortem neuropathological data. In the CSF, P-tau phosphorylated at threonine 231 (P-tau_231P) was analysed. Post-mortem, scores of neurofibrillary tangles (NFT) and neuritic plaques (NP) were assessed in frontal, temporal, parietal and hippocampal cortical areas. In the same cortical regions, load of hyperphosphorylated tau protein (HP-tau load) was determined. Concentrations of P-tau_231P were measured in frontal cortex homogenates. We found significant correlations between CSF P-tau_231P concentrations and scores of NFTs and HP-tau load in all neocortical regions studied. The score of NPs was correlated with CSF P-tau_231P only within the frontal cortex. There was a correlation between P-tau_231P in CSF and brain homogenates. These findings indicate that CSF P-tau_231P may serve as an in vivo surrogate biomarker of neurofibrillary pathology in Alzheimer's disease.