Dementia with Lewy bodies (DLB) is the second most common cause of neurodegenerative dementia in the elderly. DLB is clinically characterized by fluctuating cognitive dysfunction, visual hallucinations, and parkinsonism. It is sometimes difficult to clinically diagnose patients with DLB because of the overlap of the clinical and pathological findings of this disease with those of Alzheimer's disease. Precise clinical diagnosis is important for determining the drug therapy appropriate for improving DLB symptoms. Biomarkers that enable more accurate diagnosis of DLB and reflect its complex clinical progression are highly desired. Cognitive impairment frequently occurs in patients with Parkinson's disease (PD). The common pathological features of DLB and PD are Lewy bodies and Lewy neurites, both of which are composed of α-synuclein. DLB and PD are now considered to belong to a common spectrum of disorders designated as Lewy body disease (LBD). We have previously reported that the α-synuclein levels in cerebrospinal fluid (CSF) are significantly lower in patients with DLB than in patients with other types of dementia. However, the significance of quantifying the CSF α-synuclein levels as a biomarker for DLB has been a matter of debate because the results for these levels have been inconsistent among different studies. Similar inconsistency has been observed in studies on the plasma α-synuclein levels in patients with LBD. Here, we review the recent progress on biomarkers of LBD, especially in DLB, and discuss their application to clinical diagnosis.