Alpha-synuclein is the major constituent of Lewy bodies found in neurons in dementia with Lewy bodies (DLB) and might be of diagnostic value as a biomarker for DLB. We hypothesized that, as a consequence of increased accumulation of alpha-synuclein intraneuronally in DLB, the levels of alpha-synuclein in cerebrospinal fluid (CSF) of DLB patients would be lower than in other dementias. Our objective was to investigate the CSF levels of alpha-synuclein in several dementia disorders compared to control levels and to investigate the diagnostic value of CSF alpha-synuclein as a marker to discriminate between DLB and other types of dementia. We analyzed the levels of alpha-synuclein in CSF of 40 DLB patients, 131 patients with Alzheimer's disease, 28 patients with vascular dementia, and 39 patients with frontotemporal dementia. We did not find any significant differences in CSF alpha-synuclein levels between DLB patients and patients with Alzheimer's disease, vascular dementia or frontotemporal dementia. We conclude that in clinically diagnosed patients, alpha-synuclein does not appear to be a useful biomarker for the differentiation between DLB and other types of dementia.