Amyloid-β (Aβ) is known as the most prominent core protein in Alzheimer's Disease (AD) senile plaques. Although research has focused mainly on Aβ40 and Aβ42 as potential cerebrospinal fluid (CSF) biomarkers, a range of Aβ peptides with variable lengths has been demonstrated in the brains and CSF of AD patients. Recently, it has been found that the Aβ43 peptide may be more abundant than previously assumed, could therefore play an important role in AD pathophysiology, and hence also function as putative biomarker. In this study the value of CSF Aβ43 in AD diagnosis was investigated. Aβ43 levels in CSF were highly correlated with Aβ42 levels. Furthermore, in differentiation of AD from nondemented controls and from patients with Lewy body dementia and frontotemporal dementia, Aβ43 had an equal diagnostic value as Aβ42, both as a single biomarker and in combination with total and phosphorylated tau. In conclusion, quantification of Aβ43 in CSF does not add novel diagnostic information to the differential diagnosis of AD compared to existing biomarkers.