Cerebrospinal fluid (CSF) biochemical diagnostic markers may be valuable to help in the diagnosis early in the course of Alzheimer's disease (AD), especially in the phase before clinically overt dementia, i.e. in patients with mild cognitive impairment (MCI). We studied 44 patients with MCI who, at 1-year follow-up investigation, had progressed to AD with dementia, and 32 controls. Three CSF biomarkers related to the central pathogenic processes in AD were analysed, including CSF total-tau (T-tau) (as a marker for neuronal degeneration), CSF phospho-tau (P-tau) (as a marker for hyperphosphorylation of tau and possibly for the formation of neurofibrillary tangles), and CSF A beta 42 (as a marker for A beta metabolism, and possibly for the formation of senile plaques). At baseline, 35/44 (79.5%) of the MCI patients had high CSF T-tau, 31/44 (70.4%) high CSF P-tau, while 34/44 (77.3%) had low CSF-A beta 42 levels. The positive likelihood ratio was 8.45 for CSF T-tau, 7.49 for CSF P-tau and 8.20 for CSF A beta 42. These findings suggest that these CSF-markers are abnormal before the onset of clinical dementia, and that they may help to identify MCI patients that will progress to AD. CSF diagnostic markers will be especially important when drugs with potential effects on the progression of AD (e.g. gamma-secretase inhibitors) will reach the clinical phase.