Alzheimer's disease (AD) is the most common cause of dementia in the elderly. It is characterized pathologically by the formation of senile plaques and neurofibrilly tangles in the brain. Diagnostic markers for detecting earlier stages of AD are needed. We measured the intensity of concanavalin A (Con A) binding activities of glycoproteins of the cerebrospinal fluid (CSF) and serum of subjects to clarify the modification of core mannose since we expected that aberrant glycosylation of glycoproteins might be useful as a new bio-marker for detecting AD. CSF samples were collected from 15 patients with probable AD (AD group), 5 patients with probable dementia with Lewy bodies (DLB) (DLB group) and 8 controls without dementia (control group), whereas serum samples from 20 patients with probable AD and 20 controls without dementia were also collected. Glycoproteins in the CSF and serum were detected by lectin blotting using Con A. In the CSF of the AD group, 2 Con A binding glycoproteins were significantly higher compared with the control group. Furthermore, using analysis of variance, 3 Con A binding glycoproteins detected from the CSF of the AD group showed significant differences among the 3 groups. The levels of 3 Con A binding glycoproteins were significantly lower than in non-dementia controls in the serum. These changes in Con A binding activities did not depend on the amount of proteins. Therefore, the data indicate that the aberrance of protein glycosylation relates to the pathology of AD, and has some promise as a new biomarker for the diagnosis of AD.